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Logic Man
You guys do know that this problem has been floating around the internet for some time now, right? And it is widely known that the "riddle" aspect of it is in that most people, at least initially, think that a plane moves relative to the ground via methods similar to a car, a person, or anything that requires frictin between it and the ground in order to move. Its also widely known that the answer to the riddle is that the plane can take off. You can't prove that the plane can't take off without violating laws of Physics.

I glanced over several pages of replies here, and there are more than enough explanations of every relevant aspect of the problem, as well as some good analogies, and even a video that should allow any intelligent person to understand. If you have any Physics training, then you should know what a free body diagram is. Do one for this scenario, FBD's are an excellent tool for understanding a scenario that is confusing on the surface.
Guest
QUOTE (Mech Tech LLNL+Dec 1 2005, 06:44 PM)

Plane is on the belt, engines fired up with enough thrust to propel it forward at a speed of 100 mph. Belt moves in the opposite direction at 100 mph. If you're sitting on the belt, the plane appears to go screaming by at 100 mph. If you're in the cockpit of the plane, you appear to be moving at 100 mph along the surface of the belt.

But since the belt is run by a large mechanism that is sitting firmly on the earth, the plane's motion relative to the earth (and, most importntly, the surrounding air) is zero.

In what way am I sodomizing Sir Isaac?

The belt doesn't isolate the plane from the air. The air is the 'fluid' that the engines move, and because of the law of momentum conservation, the plane moves forward. The only effect the belt has is to add slightly to the rolling resistance of the wheels any time the plane is moving, as they are rotating at twice the speed they would be on a normal runway. This small added rolling resistance is nowhere near enough to overcome the engine thrust, so the plane can accelerate easily.
Guest
I'm not convinced either way. There are to many variables that would come into play if this experiment actually took place.

My biggest concern is a plane on solid ground would start moving sooner then a plane on a treadmill. Its ALOT easier moving an object on wheels.

Imagine a plane weighs 10 tons and its suspended in the air. It would take 10 tons of force to begin to start moving.
Now imagine a plane on the ground with just three sets of wheels under it.
50 midgets or an elephant could start moving this plane (lol).

So I think that a small enough jet (like a Blue Angels) would take off because of their power to weight ratio.
A passenger jet would run out of power before it gained enough speed to get off the ground.

Now the real question that should start a larger debate.
If a guy sucks one cock does that make him gay or just a cocksucker?
Guest
QUOTE (Guest+Dec 1 2005, 07:13 PM)
I'm not convinced either way. If a guy sucks one cock does that make him gay or just a cocksucker?

You're both gay and a cocksucker. Sorry for the bad news.
blimpie
Man when the laws of common sense finally make their way back around I will feel very embarrased for Mech Tech, LLNL.

Bottomline my uppity friend is that this is not an open question, but is a riddle to confuse. You have been confused. The mere fact that you equate walking on a moving conveyor to a jet on a conveyor is quick sad, the fact that you seem to talk down to everyone else makes it funny.
Insyght
Mech Tech, LLNL,

Isn't it amazing that they cannot "see"?

Simply amazing.

For plane lift, need forward movement through air.
If plane is not moving through air, because it is restrained through some conveyor system, then no lift will occur.

How simple could it possibly be?

And they accuse me of smoking the splicy herbals?
Guest
QUOTE (Insyght+Dec 1 2005, 07:16 PM)
And they accuse me of smoking the splicy herbals?

Ok, maybe you're just stupid. That's ok though, it makes the world a more interesting place.
Mech Tech LLNL
QUOTE (Rebes+Dec 1 2005, 06:54 PM)

QUOTE
This is amazing.  Once again you are stating that the belt is moving at some speed to hold the plane stationary

I'm not saying that- the originator of the problem is. Look:

QUOTE (->
 QUOTE This is amazing.  Once again you are stating that the belt is moving at some speed to hold the plane stationary

I'm not saying that- the originator of the problem is. Look:

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

So, <sigh> one more time: Equal and Opposite motions (should sound familiar) resulting in a net motion of (drumroll) ZERO. Plane's engines screaming, plane's wheels rolling, belt motors revved up, belt rolling, hamster running his little heart out in his wheel and NOBODY moving relative to the earth and surrounding air despite a whole lot of energy being converted into motion.

QUOTE
yet not realizing that because the plane is stationary, the belt should not be moving, per the original poster's description of the problem.

Dude, the reason the plane IS stationary relative to the earth is BECAUSE the belt is moving. Go back to the "walking the wrong way on the people mover" example. Go to Macy's and walk the wrong way on the escalator. Lots of motion and moving parts, zero motion relative to your surroundings.

Hey, here's a picture to make it clearer:

QUOTE (->
 QUOTE yet not realizing that because the plane is stationary, the belt should not be moving, per the original poster's description of the problem.

Dude, the reason the plane IS stationary relative to the earth is BECAUSE the belt is moving. Go back to the "walking the wrong way on the people mover" example. Go to Macy's and walk the wrong way on the escalator. Lots of motion and moving parts, zero motion relative to your surroundings.

Hey, here's a picture to make it clearer:

Do not even say that the conveyor is in motion because the plane "looks like it's moving, from the point of view of the conveyor."

Again, I'm not the one who's saying it. See the original problem. I'm kind of at a loss as how to make this any clearer.

QUOTE
What we are talking about is the conveyor moves backwards when the plane moves forward through the air.

The plane has to get airborne first. Something that ain't going to happen when it's on a treadmill that holds it stationary relative to the surrounding air.

Okay, question for you- what exactly is the purpose of "the conveyor mov(ing) backwards when the plane moves forward through the air."?

Rebes
QUOTE (Insyght+Dec 1 2005, 07:16 PM)
Mech Tech, LLNL,

Isn't it amazing that they cannot "see"?

Simply amazing.

For plane lift, need forward movement through air.
If plane is not moving through air, because it is restrained through some conveyor system, then no lift will occur.

How simple could it possibly be?

And they accuse me of smoking the splicy herbals?

Insyght, we have explained several times that in fact the plane does move forward through the air.

Explain to me exactly how this plane sits still? We have done our part to explain exactly how it moves forward.

Once again, since it seems you are not reading any of my posts, and just replying with the typical "You're stupid and it doesn't fly because I say so" attitude:

If the plane is being held in place by a moving conveyor belt, but the question plainly states that the belt moves backwards at the speed that the plane is moving forwards, why is the conveyor moving to hold the plane in place? Should the conveyor not be stopped because the plane is sitting in one spot?

Someone stated that initial reactions to this problem are that it sits still, but once you see why it moves, it makes total sense. They also said that many people have changed their answer from "No, it doesn't." to "Yes, it does." after seeing what's going on... But not one person goes from "Yes" to "No". Why do you think this is? Someone also stated this riddle has been around for a long time, and that it plays against people's initial instincts for what they believe to be true. Both statements also quite true.

Rebes.
Mech Tech LLNL
QUOTE (blimpie+Dec 1 2005, 07:15 PM)
Man when the laws of common sense finally make their way back around I will feel very embarrased for Mech Tech, LLNL.

Bottomline my uppity friend is that this is not an open question, but is a riddle to confuse. You have been confused. The mere fact that you equate walking on a moving conveyor to a jet on a conveyor is quick sad, the fact that you seem to talk down to everyone else makes it funny.

blah blah blah....

I see rhetoric. And nothing that even remotely resembles a reasoned, practical, logical proof (or even a theory).

I welcome any counter to my proposals, as long as they are rooted in math and physics. Anything else is just mental masturbation.

Oh, and common sense once held that the earth was flat, and the center of the universe.
Sage
QUOTE (Mech Tech LLNL+Dec 1 2005, 06:44 PM)
[QUOTE=Rebes,Dec 1 2005, 06:06 PM]

You could have the plane's engines screaming at full throttle and it rolling at 1,000 mph relative to the belt.

Stick with this bit and you're almost there. If the belt and plane have a relative speed of 1000mph, each must be travelling at 500mph (remember the puzzle - the belt travels at exactly the same speed as the plane but in the opposite direction). At 500mph the plane is cruising never mind taking off!
Another_Guest
Mech Tech - How is the conveyor "holding back" the plane and causing it to remain stationary ?
The engines are pushing against the air thrusting the plane forward, while the conveyor is merely spinning the planes wheels. The wheel SPINS - there is no power or drag imparted to the plane.
Rebes
QUOTE (Mech Tech LLNL+Dec 1 2005, 07:33 PM)
I'm not saying that- the originator of the problem is.  Look:

QUOTE

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

So, <sigh> one more time: Equal and Opposite motions (should sound familiar) resulting in a net motion of (drumroll) ZERO. Plane's engines screaming, plane's wheels rolling, belt motors revved up, belt rolling, hamster running his little heart out in his wheel and NOBODY moving relative to the earth and surrounding air despite a whole lot of energy being converted into motion.

The original poster says the conveyor moves backwards at the same speed that the plane moves forward. It does not say that the conveyor works like crazy to hold the plane in one place.

Is this so hard to fathom: If the plane is in one place, why is the conveyor spinning?!?!?!

Next, your theory of Equal and Opposite is flawed. In order for that to hold true, the Equal and Opposite must be along the same line. The "Opposite" that you speak of is merely rolling against the wheels of the plane, while the initial force is pushing directly on the back of the plane.

If you use your theory, take a 4x4, 6 foot long piece of lumber and stand it on end. Push forward on the top with a 5 kg force, and at the same time, push backwards on the bottom with a 5 kg force. HEY! Equal and opposite!!! The 4x4 does not move! WRONG! The forces are along different lines, and therefore do NOT cancel each other out. The board topples over onto it's side, exactly as we logically expect.

Edit: The opposite "reaction" to the force of the engines is that the plane moves forward.

Rebes.
Insyght
QUOTE
Something that ain't going to happen when it's on a treadmill that holds it stationary relative to the surrounding air

Exactly the point.... take note everyone.... H .. O .. L .. D .. S .. I .. T .. S .. T .. A .. T .. I .. O .. N .. A .. R .. Y.

IF the belt, HOLDS the plane, it will not take off.
IF the belt just spins become it would look pretty but has no HOLD on the plane, then the plan will simply fly off the end of the belt.

QUOTE (->
 QUOTE Something that ain't going to happen when it's on a treadmill that holds it stationary relative to the surrounding air

Exactly the point.... take note everyone.... H .. O .. L .. D .. S .. I .. T .. S .. T .. A .. T .. I .. O .. N .. A .. R .. Y.

IF the belt, HOLDS the plane, it will not take off.
IF the belt just spins become it would look pretty but has no HOLD on the plane, then the plan will simply fly off the end of the belt.

why is the conveyor moving to hold the plane in place? Should the conveyor not be stopped because the plane is sitting in one spot?

Lets re-visit the operation of the belt:

QUOTE
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

First note that the plane is "standing". Doesn't that signify to you a non-moving plane?

Second note that as the plane "moves" in one direction the conveyor moves in opposite direction ... it "tracks" the plane speed and adjusts the speed of the conveyor to exactly match, meaning that the plane is accelerating...

The question always assumes that the plane is ON the belt, otherwise how could it adjust it's speed to match the plane?

Therefore it assumes that the belt movement is COUNTERACTING the movement of the plane.

If you do not get the same anaylsis from this riddle, please comment on how you interpret this text. I have outlined in detail I derived from it.

Our disagreements I feel are related simply to "unstanding" the riddle.
Mech Tech LLNL
QUOTE (Guest+Dec 1 2005, 07:12 PM)

QUOTE
The belt doesn't isolate the plane from the air.

Never said it did. Engines suck in air, expel superheated stream of air and burned fuel particles at an astonishing rate, creating thrust. Thrust pushes against the air.

Then what happens?

The plane begins to roll along the belt. And the belt matches the plane's speed, but in the opposite direction. And around and around we go....

QUOTE (->
 QUOTE The belt doesn't isolate the plane from the air.

Never said it did. Engines suck in air, expel superheated stream of air and burned fuel particles at an astonishing rate, creating thrust. Thrust pushes against the air.

Then what happens?

The plane begins to roll along the belt. And the belt matches the plane's speed, but in the opposite direction. And around and around we go....

The only effect the belt has is to add slightly to the rolling resistance of the wheels any time the plane is moving, as they are rotating at twice the speed they would be on a normal runway.  This small added rolling resistance is nowhere near enough to overcome the engine thrust, so the plane can accelerate easily.

You're repeating yourself. And at the risk of doing so as well, it doesn't matter that the plane can accelerate. It's supposed to accelerate. Sir Isaac would be upset if it didn't accelerate. But for the umpteenth frigging time, READ THE FRIGGING ORIGINAL PROBLEM AND UNDERSTAND WHAT IT SAYS. The plane's motion is cancelled out by the belt's motion.

Okay, here's a different example. Go back to the picture of the guy on the treadmaill.

Put him on roller skates. Imagine he ate 8,973 bean burritos for lunch. He's got gas so bad that he emits a jet of flattus that creates a continuous thrust of 100 pounds. This is enough thrust to propel him forward. Once he starts rolling forward, turn on the treadmill. Match his forward speed with the treadmill's speed. He remains on the treadmill. His motion relative to the treadmill's belt can be pretty high- after a half an hour of 100 pounds of continuous thrust, you've got the treadmill moving at 3,000 miles per hour to match his speed, yet he remains on the treadmill. His motion relative to his surroundings is zero.

Awwww, screw it.

I got WAY better stuff to be doing with my time.
SkiSTX
QUOTE
Our disagreements I feel are related simply to "unstanding" the riddle.

Insyght, you are exactly right. Us "yes it will fly" people believe that the statement "The plane moves in one direction" means it literally MOVE RELATIVE TO THE GROUND... not relative to the belt.

But this was not explicitly stated in the OP which, alas is the cause of the confusion....

If you take the planes movement to mean RERELATIVE TO THE GROUND and the conveyor's movement to mean RELATIVE TO THE GROUND, would you agree that the plane would then take off?
Rebes
QUOTE (Mech Tech LLNL+Dec 1 2005, 07:57 PM)
Put him on roller skates. Imagine he ate 8,973 bean burritos for lunch. He's got gas so bad that he emits a jet of flattus that creates a continuous thrust of 100 pounds. This is enough thrust to propel him forward. Once he starts rolling forward, turn on the treadmill. Match his forward speed with the treadmill's speed. He remains on the treadmill. His motion relative to the treadmill's belt can be pretty high- after a half an hour of 100 pounds of continuous thrust, you've got the treadmill moving at 3,000 miles per hour to match his speed, yet he remains on the treadmill. His motion relative to his surroundings is zero.

Awwww, screw it.

I got WAY better stuff to be doing with my time.

Carry this through Mech, you're on the right track...

You said his motion relative to his surroundings is zero. If the speed of the conveyor is matched to his speed relative to his surroundings, then the speed of the conveyor is zero, not 3,000 miles per hour.

The speed of the conveyor matches his speed relative to his surroundings. Simple.

Well done!

Rebes.
Insyght
QUOTE
If you take the planes movement to mean RERELATIVE TO THE GROUND and the conveyor's movement to mean RELATIVE TO THE GROUND, would you agree that the plane would then take off?

If the plane is moving relative to the ground (meaning the whole conveyor belt = nothing) then yes, the plane would move along the length of the conveyor belt. If it is short, it would drop off the end and carry on down the road. If it is long, the plane would travel down it and eventually reach lift speed and take off.

Do you agree however, that if the belt was compensating or holding the plane (somehow as the riddle implies) that the plane would not take off?
Guest
Insyght and Mech Tech LLNL! Kudos to you both!

Your acumen for the spurious and passion for the untenable is a fine and true tribute to all that is ludicrous! To argue against either of you is nothing short of futile given the commitment you both have in justifying the illegitimate! In fact, since at this point only a nincompoop would dare disagree with you, I in turn salute you!
When reason itself has become exasperated, only a halfwit would continue attempts of persuasion along the lines of the rational. This does indeed make your opponents, though innocent they may be, imbeciles in the grandest sense.

And so this brings about the oft overlooked condition of reason and idiocy occupying the same mind at the same time. For that we should all be thankful! I join you in your crusade....The plane can not fly!!
Rebes
QUOTE (Insyght+Dec 1 2005, 07:51 PM)
Lets re-visit the operation of the belt:

QUOTE
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

First note that the plane is "standing". Doesn't that signify to you a non-moving plane?

If you want to use simple grammar to twist this into whatever you'd like, I'd also like to mention that in the same statement by the OP, he also said that "The plane moves in one direction..."

Stop looking at the grammar. So what if the plane is standing to begin with? Then, the engines kick in, and all hell breaks loose!

Let's try this: Truck (in neutral). Conveyor. Chain. Super strong chain on front bumper of truck. Chain attached at far end of conveyor (not on conveyor) to a winch. Winch begins to pull the truck forward by winding in the chain. Truck is moving forward at 1 mph, conveyor is moving backwards at 1 mph. Wheels are spinning at 2 mph. Truck is most definitely moving forward. Chain is getting shorter. Truck aproaches winch.

Now replace the winch and chain with a jet engine on the back side of the truck, because that's all the chain/winch is... a force not linked to the wheels or conveyor. I trust you believe as well as I that a jet engine is a very powerful thing. It's fully capable of pushing that truck forward the same way that the chain pulled it forward. Now replace the truck with the airplane. The airplane moves forward.

Rebes.
SkiSTX
QUOTE
Do you agree however, that if the belt was compensating or holding the plane (somehow as the riddle implies) that the plane would not take off?

Well, I think to hold the plane stationary, the conveyor would have to rotate at a million miles an hour to put enough friction on the wheels to counteract the force of engines pushing against stationary air. But I don't think this is what the OP meant by 'conveyor moves backwards at the same speed plane moves forward'.

Again, I think it is a difference of how you read the question. During the first few pages of this discussion I was a 'no fly' person. But then I went back and re-read the OP coming to the conclusion that 'move forward' and 'move backward' can only really mean 'relative to the earth'. I was converted from the dark side after that point!
Logic Man

Ok folks. Here an FBD showing all the forces acting on the plane in this problem. These same forces act on the plane whether the 'runway' is moving or not. Lets use Ft for the thrust force, Fd for the drag force, and Fr for the rolling resistance force.

On a stationary runway Ft > Fd + Fr. If you know that planes can take off from a normal runway, you'll agree with this.

If the runway is moving in the opposite direction of the plane, it doesn't change Fd. It does make Fr increase some. The runway is moving in the opposite direction of the planes movement (not 'countering' its movement). Since it moves at the same speed of the plane in the opposite direction, the wheels rotate at twice their normal rate. This does not add enough rolling resistance to make Ft = Fd + Fr, or to make Ft < Fd + Fr. This means that the plane will still accelerate forward. Unless you can tell us how Fr gets so big to allow the sum Fr + Fd to meet or exceed Ft, then this will hold true.

Once you understand that, the rest is a foregone conclusion. We all know that if the plane can accelerate almost as easily as it will on a normal runway, it will be able to take off. During the takeoff transition, the normal force will decrease and the lift force will increase, until the lift force exceeds that of gravity on the plane and the normal force goes to zero. At that point, the plane has officially taken off.

No amount of theorizing, imagining, or arguing can refute this. As the problem is stated, it can take off.
Logic Man
QUOTE (SkiSTX+Dec 1 2005, 07:58 PM)
But this was not explicitly stated in the OP which, alas is the cause of the confusion....

If you take the planes movement to mean RERELATIVE TO THE GROUND and the conveyor's movement to mean RELATIVE TO THE GROUND, would you agree that the plane would then take off?

Since the speed of the plane and the speed of the belt are supposed to match, they would have to use the same frame of reference for measurement of that speed. The ground would be a good reference to use for measurement of both their speeds. Using the ground as the reference for the belt, but the belt as the reference for the plane doesn't allow you to compare their speeds in the context of the problem.
Logic Man
QUOTE (Guest+Dec 1 2005, 08:08 PM)
Insyght and Mech Tech LLNL! Kudos to you both!

Your acumen for the spurious and passion for the untenable is a fine and true tribute to all that is ludicrous! To argue against either of you is nothing short of futile given the commitment you both have in justifying the illegitimate! In fact, since at this point only a nincompoop would dare disagree with you, I in turn salute you!
When reason itself has become exasperated, only a halfwit would continue attempts of persuasion along the lines of the rational. This does indeed make your opponents, though innocent they may be, imbeciles in the grandest sense.

And so this brings about the oft overlooked condition of reason and idiocy occupying the same mind at the same time. For that we should all be thankful! I join you in your crusade....The plane can not fly!!

Thanks for the laugh
MassiveAttack
QUOTE (Mech Tech LLNL+Dec 1 2005, 07:38 PM)
QUOTE (blimpie+Dec 1 2005, 07:15 PM)
Man when the laws of common sense finally make their way back around I will feel very embarrased for Mech Tech, LLNL.

Bottomline my uppity friend is that this is not an open question, but is a riddle to confuse.  You have been confused.  The mere fact that you equate walking on a moving conveyor to a jet on a conveyor is quick sad, the fact that you seem to talk down to everyone else makes it funny.

blah blah blah....

I see rhetoric. And nothing that even remotely resembles a reasoned, practical, logical proof (or even a theory).

I welcome any counter to my proposals, as long as they are rooted in math and physics. Anything else is just mental masturbation.

Oh, and common sense once held that the earth was flat, and the center of the universe.

Trying to prove something simple like this to you over the internet is just as difficult as it is to prove to you that the world is round.

You have been given proof. You are obviously just trying to stir up this loosing debate.
Insyght
QUOTE
Thanks for the laugh

Your welcome. Just watch out for them spicy herbals. Not good for you.

Rebes,

You have wimped out on your full description of how you understand the conveyor belt in the riddle. You merely pick on my use of grammer regarding "standing".

Fine. Not got a problem with that.

Explain the purpose of the belt in the riddle?

Is it simply to apply a small resistance as Mr.Logic.Man has represented by Fr in his diagram?

If the quesiton poster intended to only apply medeocre resistance then why have it accelerate with the plane?

Don't wimp out on the obviously implied (not clearly stated) purpose of the belt.

The poster was clearly picturing a SHORT belt, with a plane on it, spinning on the spot, belt speeding up, faster and faster, whilst the plane just sits there, until the plane starts to lift off the floor.

Explain the purpose of the belt as you see it.

InSyght - processing the greenies.
Sage
QUOTE (Insyght+Dec 1 2005, 09:05 PM)
QUOTE
Thanks for the laugh

Your welcome. Just watch out for them spicy herbals. Not good for you.

Rebes,

You have wimped out on your full description of how you understand the conveyor belt in the riddle. You merely pick on my use of grammer regarding "standing".

Fine. Not got a problem with that.

Explain the purpose of the belt in the riddle?

Is it simply to apply a small resistance as Mr.Logic.Man has represented by Fr in his diagram?

If the quesiton poster intended to only apply medeocre resistance then why have it accelerate with the plane?

Don't wimp out on the obviously implied (not clearly stated) purpose of the belt.

The poster was clearly picturing a SHORT belt, with a plane on it, spinning on the spot, belt speeding up, faster and faster, whilst the plane just sits there, until the plane starts to lift off the floor.

Explain the purpose of the belt as you see it.

InSyght - processing the greenies.

The puzzle states a runway that can move - how does this imply a short belt? All the runways I've seen are pretty big.

Nothing wrong with your understanding of 'standing', but then you seem to disregard your dictionary when you interpret 'it moves'. It's perfectly logical to suppose a plane is stationary prior to it's take off and that it then starts to move in order to take off.

The purpose of the belt in the riddle is solely to confuse and befuddle - that's why it makes an entertaining puzzle, because at first glance the answer is counter-intuitive.
Mech Tech LLNL

QUOTE

The runway is moving in the opposite direction of the planes movement (not 'countering' its movement).

This is the heart of the matter, and the point where the argument falls to either side (the plane will or will not take off)

Nice FDB, by the way.

Here's my last post on this subject, and I'm going to take a different tack.

First, the OP:
QUOTE (->
 QUOTE The runway is moving in the opposite direction of the planes movement (not 'countering' its movement).

This is the heart of the matter, and the point where the argument falls to either side (the plane will or will not take off)

Nice FDB, by the way.

Here's my last post on this subject, and I'm going to take a different tack.

First, the OP:
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

So let's play with a 747.

I think we're all on the same page as far as the plane needing to have motion relative to the surrounding air in order to have lift. If the plane isn't moving at, say, 150 mph relative to the surrounding air, the wings can't generate sufficient lift to overcome gravity.

So let's put the 747 on our big treadmill-like belt. With no inputs (jet engines, belt motors, etc) it just sits there. Gravity holds it to the belt's surface, and we have no lift to counteract gravity.

Now let's tie one end of a really strong rope to the nose gear of the 747, and the other end to a nice, sturdy building. Making sure the 747's brakes are released, we set the belt into motion. Let's run it up to, say, 10 miles per hour.

As soon as the belt starts moving, the 747 moves backward along with it. That is, until the slack is taken out of the rope. Now the 747's tires roll. We notice there's a fair amount of tension in the rope. This is due to the rolling resistance force as shown in that nice FBD. Now we fire up the 747's engines and produce -just- enough thrust to take the tension out of the rope. The propulsive forces of the engines are -just- counteracting the rolling resisitance force. Note that there is no drag force- though the plane is rolling at 10 mph relative to the belt, it is stationary relative to the surrounding air; no aerodynamic drag is produced. Nor is there any lift generated, for the same reason.

So now let's crank up the belt's speed to 100 mph. The rope goes very tight again, due to rolling resistance. If we let the system become stable (running the belt at 100 mph for a few minutes), then cut the rope, the 747 would move backwards. It has inertia relative to the belt, but rolling resistance forces would slow the plane relative to the belt, and it would slowly move toward the beck end of the conveyor system. So to counter that, let's crank up the engines some more so that the plane is once again stationary relative to us, and moving at 100 mph relative to the belt.

You can see where this is going. We can continue to increase the belt's speed, and counter it with the engines' thrust so that the 747 remains stationary relative to our position, and to the surrounding air. It doesn't make any difference if we run the belt to a speed that exceeds the 747's minimum takeoff velocity- since the plane isn't moving relative to the surrounding air, there is no lift.

So this scenario is just a minor twist on the OP. We set the belt into motion and counter it with thrust, instead of applying thrust to the plane and countering it with motion.

In both cases, gravity remains operational. And no lift is generated as long as the plane remains stationary, which is assured by the balancing of rolling resistance (a function of gravity and belt motion) against thrust.

Do note that because there is no aerodynamic drag, the plane will be able to achieve some pretty spectacular speed relative to the belt- much higher speeds that it could attain on, say, a 1000 mile long runway. And at some point, the wheel bearings will meet some physical limits of drag (you can't spin a bearing to an infinite speed), so, practically speaking, you'd eventually arrive at the point where the engines were maxed out on thrust and the plane couldn't go any faster relative to the belt- bearing drag forces would rise exponentially as they met their speed limits.

I'm signing off now. The only reason I've spent as much time here as I have is that I'm at home, sick with a really nasty cold. Sorry if I've stepped on anyone's toes in my virally-influenced mood.

Respect Newton!
Guest
QUOTE (Guest+Nov 4 2005, 06:53 AM)
here's a simple test you can do:
tape a piece of sting to a car and place the car on a piece of paper then pull the string forward......the car moves forward
now place the car back on the paper and pull the string forward while pulling the paper backwards at the same speed or faster if you like....the car still move forward.

the propellers on the plane are pulling the plane forward through the air. The thrust is applied to the air! Since the air is stationary and since the propellers are applying thrust on the air...not the belt, the plane moves forward.
The air, like the string, is the method of propulsion, and the method of propulsion in both these cases is independent of the conveyorbelt/paper.
If you wanted to keep the plane from moving forward while it's propellers are going you'd have to manipulate the air. For example if the propellers were giving a 10mph thrust and there was a 10 mph head wind, the plane would stay in the same place.
If the method of propulsion came from contact with the ground (propulsion is applied to the ground), as with a normal car, then manipulating the ground (ie. with a conveyerbelt) would manipulate the result of the propulsion..that is its forward motion. you could even perfectly cancel out the cars forward motion by manipulation the ground.
Since the plane gets it's propulsion via the air, to keep it from moving forward you'd have to manipulate the air...not the ground.

What the!!??
Who'd of guessed!

It's been explained already! Boy do I have egg on my face! Why didn't I read these explanations before? I feel like an idiot for not taking on the simple chore of reading previous posts and thinking clearly before I wasted so much time trying to explain why it won't fly.

And Geez!, It's been explained by so many people almost right from the beginning!

Like this post for example by "fmbqa" way back in July!

"Yes, the plane will take off, practically as it normally would. If you couldn't see the "runway" moving, you would see no difference to a normal takeoff, unless you could discern that the plane's wheels were spinning twice as fast as normal.

The reason: The plane, even if on the ground, accelerates relative to the AIR! Thanks to its wheels there is very little friction between it and the ground it stands on, so what the ground does is of almost no importance.

Now, if you would put wings on a car, that would be different. The takeoff would not work, because the car uses its wheels to push itself forward relative to the GROUND. It would not achieve any speed relative to the air, so its wings could create no lift."

or so many others like it.

Oh, well. I guess my only out now is to steadfastly stick to my guns and to muddy the waters until everyone else simply get's frustrated or bored and goes away. Hmmmm. That'll do just fine....they'll go away eventually. Yeah...That'll just leave me...the victor! Alone in my beautiful motionless plane! MUuAHAHAHAaa!...FOoools!

Guest
QUOTE (Insyght+Dec 1 2005, 09:05 PM)
Explain the purpose of the belt in the riddle?

Is it simply to apply a small resistance as Mr.Logic.Man has represented by Fr in his diagram?

If the quesiton poster intended to only apply medeocre resistance then why have it accelerate with the plane?

Don't wimp out on the obviously implied (not clearly stated) purpose of the belt.

The poster was clearly picturing a SHORT belt, with a plane on it, spinning on the spot, belt speeding up, faster and faster, whilst the plane just sits there, until the plane starts to lift off the floor.

Explain the purpose of the belt as you see it.

InSyght - processing the greenies.

I'll break it down for ya. Here is the original question:

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

------------------------------------------------------------------------------------------

"A plane is standing on runway that can move (some sort of band conveyer)."
-This sets up the initial state of the scenario. Nothing very complicated.

"The plane moves in one direction, while the conveyer moves in the opposite direction."
-Ok, that makes sense. If the plane moves to the right as viewed by an observer on the stationary ground, then the belt surface moves to the left.

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."
-Now, the plane's speed and the belt's speed are being compared, so they need a common reference. Let's use the ground again, no need to complicate things. So if the plane moves at 1mph to the right, the belt surface moves at 1mph to the left. If the plane moves at 100 mph to the right, the belt surface moves at 100mph to the left. Still pretty straightforward.

There is nothing there that says the belt holds the plane in one place. There's not even anything there that implies that. The problem is actually pretty clear. It doesn't mention wind speed, so ignore it....make it 0. Other things that aren't specifically mentioned that would effect this include the rolling resistance characteristics, the amount of thrust available, the weight of the plane, the airspeed needed for takeoff (Vrotate)...but we don't need these because it isn't a math problem such as "how far will the plane travel before taking off?". All we want is a yes/no answer, and in this problem that can be arrived at with the given information. Any plane that is able to take off in conditions less than perfect (such as a tailwind) will be able to take off in the given scenario because there is nothing happening that will stop if from accelerating. It will simply accelerate a little bit more slowly than normal.
Insyght
QUOTE
The purpose of the belt in the riddle is solely to confuse and befuddle - that's why it makes an entertaining puzzle, because at first glance the answer is counter-intuitive.

Thanks. So Sage makes his understanding clear. The belt as he understands it is a complete coy, to try and confuse.

Anyone else agree?

Wonder if dirak is still around?
Sage
Mech Tech, don't give up yet. You will be rewarded when you get it.

Everything you say is exactly right. Unfortunately, you are failing to answer the actual question that was asked.

Your fundamental stumbling block is your refusal to accept that the plane moves - something the question states categorically. It does not state that the wheels rotate to counteract the movement of the belt (= stationary plane). When the plane moves one way the belt moves the other way. Therefore, the freely spinning wheels will rotate at twice the speed they would normally have to in order to accomodate the planes movement along the belt.
Logic Man
QUOTE (Guest+Dec 1 2005, 09:35 PM)
QUOTE (Insyght+Dec 1 2005, 09:05 PM)
Explain the purpose of the belt in the riddle?

Is it simply to apply a small resistance as Mr.Logic.Man has represented by Fr in his diagram?

If the quesiton poster intended to only apply medeocre resistance then why have it accelerate with the plane?

Don't wimp out on the obviously implied (not clearly stated) purpose of the belt.

The poster was clearly picturing a SHORT belt, with a plane on it, spinning on the spot, belt speeding up, faster and faster, whilst the plane just sits there, until the plane starts to lift off the floor.

Explain the purpose of the belt as you see it.

InSyght - processing the greenies.

I'll break it down for ya. Here is the original question:

A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

------------------------------------------------------------------------------------------

"A plane is standing on runway that can move (some sort of band conveyer)."
-This sets up the initial state of the scenario. Nothing very complicated.

"The plane moves in one direction, while the conveyer moves in the opposite direction."
-Ok, that makes sense. If the plane moves to the right as viewed by an observer on the stationary ground, then the belt surface moves to the left.

"This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."
-Now, the plane's speed and the belt's speed are being compared, so they need a common reference. Let's use the ground again, no need to complicate things. So if the plane moves at 1mph to the right, the belt surface moves at 1mph to the left. If the plane moves at 100 mph to the right, the belt surface moves at 100mph to the left. Still pretty straightforward.

There is nothing there that says the belt holds the plane in one place. There's not even anything there that implies that. The problem is actually pretty clear. It doesn't mention wind speed, so ignore it....make it 0. Other things that aren't specifically mentioned that would effect this include the rolling resistance characteristics, the amount of thrust available, the weight of the plane, the airspeed needed for takeoff (Vrotate)...but we don't need these because it isn't a math problem such as "how far will the plane travel before taking off?". All we want is a yes/no answer, and in this problem that can be arrived at with the given information. Any plane that is able to take off in conditions less than perfect (such as a tailwind) will be able to take off in the given scenario because there is nothing happening that will stop if from accelerating. It will simply accelerate a little bit more slowly than normal.

That was me above by the way
Rebes
Personally, I'm bored with it. I know the answer.

I have gone over this problem with an assortment of people... from people with technical degrees in mechanics, bachelor degrees in engineering, masters degrees in engineering, doctorates of engineering, pilots, and several friends. We can all see the answer.

If you are still on the "No" side, I'd suggest you link this forum to several of your friends/parents/teachers, and have them read it over.

I have explained how this works so many times that I just can't keep at it anymore. I'm not going to keep retyping the same story over and over while Insyght and Mech Tech post things like "You have wimped out on your full description of how you understand the conveyor belt in the riddle. You merely pick on my use of grammer regarding "standing".". (I'm not picking on your use of grammer, I'm picking on the fact that you use the word "standing" to mean it's always still, when in the same sentence it actually says the plane moves).

And don't say I'm wimping out, I've spent the last 5 pages of this forum trying to explain it. I've gone over it over and over. If you want me to keep explaining it, just go back and read over my posts.

Have fun everyone!

Rebes
Sage
QUOTE (Rebes+Dec 1 2005, 09:41 PM)
Personally, I'm bored with it. I know the answer.

I have gone over this problem with an assortment of people... from people with technical degrees in mechanics, bachelor degrees in engineering, masters degrees in engineering, doctorates of engineering, pilots, and several friends. We can all see the answer.

If you are still on the "No" side, I'd suggest you link this forum to several of your friends/parents/teachers, and have them read it over.

I have explained how this works so many times that I just can't keep at it anymore. I'm not going to keep retyping the same story over and over while Insyght and Mech Tech post things like "You have wimped out on your full description of how you understand the conveyor belt in the riddle. You merely pick on my use of grammer regarding "standing".". (I'm not picking on your use of grammer, I'm picking on the fact that you use the word "standing" to mean it's always still, when in the same sentence it actually says the plane moves).

And don't say I'm wimping out, I've spent the last 5 pages of this forum trying to explain it. I've gone over it over and over. If you want me to keep explaining it, just go back and read over my posts.

Have fun everyone!

Rebes

Cheers Rebes.

We tried, we failed - doubtless in three months time we could come back to this and find the same arguments raging amongst a new set of proponents, still no further on. (Just as others before us, doubtless have).

Still I enjoyed the debate, frustrating though it was.
Insyght
QUOTE
And don't say I'm wimping out, I've spent the last 5 pages of this forum trying to explain it. I've gone over it over and over. If you want me to keep explaining it, just go back and read over my posts.

Have fun everyone!

Woo Hoo, I win!

Hahaha

Na, joking aside, you are right. The plane will fly. (LOL) It was wicked, winding you guys up (LOL).

MassiveAttack
duplicate post below
MassiveAttack
QUOTE (Sage+Dec 1 2005, 09:50 PM)

Cheers Rebes.

We tried, we failed - doubtless in three months time we could come back to this and find the same arguments raging amongst a new set of proponents, still no further on.  (Just as others before us, doubtless have).

Still I enjoyed the debate, frustrating though it was.

The debate was productive. I found this website when trying to find a better explanation to post on another website. I really like the forums and will probably hang around. The funny thing is everyone on the other site finally came around, but on a physics web forum there is still debate. Go figure.

I must admit I am not as calm as you and rebes are. I get frustrated when people are staring something in the face but just can't see it. Like how the Mech Tech relates the rolling resistance of wheels on a jet to the amount of thrust of the engines..

PS. Mech Tech the rope example was supposed to show you that if you stood on a moving conveyor with skates that the conveyor would cause you to move backwards (as you show in your example) however the point you are missing is the rope is not attached to wall, it is being held by someone not on the conveyor who is walking the opposite way the conveyor is going. If that person walks at 10mph then the person with the skates on will move at 10mph regardless of the speed of the conveyor within normal means (conveyor not moving at 500mph)

In the example below (quick drawing, but you get the idea). Even though the conveyor is spinning 10mph in the opposite direction the person on the skates will still get pulled off the conveyor (forward momentum) because the person pulling the skater (jet engines in the original example) are independent of the conveyor
Example
Mr. Revenge
Ok, one last point here for Mech or anyone else with the wrong view.
It's a test, one anyone should be able to do fairly easily.

First, image a treadmill. The belt is not moving.

Second, you place a skateboard on top of it. It is not moving either.

Now, you turn the treadmill on and the belt moves backwards at 5 ft/second.
At the same time you push the skateboard forward at 5 ft/second.

Your hand == thrust from plane engine.

Mech, if you hold by your claim, you would have to admit that even when you are pushing the skateboard forward at 5 ft/second when the belt is moving backwards at 5 ft/second, it wouldn't move. That is a logic impossibility. The skateboard IS moving forward, relative to the ground, at 5 ft/second. The belt IS moving backwards, relative to the ground, at 5 ft/second.

The wheels are moving at 10 ft/second. The airplane problem does not give any specification on how fast the wheels are moving in relation to anything. It ONLY describes the exact situation with the skateboard. When the plane is moving forward at 5 ft/second, the belt is moving backwards at 5 ft/second. The plane is STILL moving forward at 5 ft/second.

Go ahead, and try it. Turn on a treadmill with a skateboard on it. Hold the skateboard in place. No problem right? It's probably a little tough to hold it there, because of a little friction, but nothing bad. Now, with the treadmill moving, push the skateboard forward slowly. SLOWER than the belt.

Not hard either, is it? According to you, because the skateboard is only moving forward slowly, and the belt is moving quickly backwards, the skateboard should fly backwards along the belt. But it doesn't, it keeps moving forward.

Since your hands are only exerting a force on the skateboard, they could be rocket engines, that makes no difference. You can't hold by your argument without destroying the laws of physics.

Hopefully that will help. Even though I'm pretty sure Mech is trolling as no one who has ever taken a physics course could possibly be that dense, hopefully other people who were not convinced and only read this thread learned something from it.
j6p
Hello Mr. Revenge, As I said earlier I did a complete 180 several pages ago. I was so sure that it wouldn't fly but logic finally prevailed. But then sometimes something is posted that makes me re-think my position and your post is it for today. So without being argumentative I would like to ask, what happens if you just put your finger behind the skate board. Not pushing it forward, just stopping it from going backwards. That would be energy expended but no movement of the board. The skate board would stay in place so this makes me think that the plane would stay in place if just enough engine power was applied to keep it there. No energy would be applied to the wheels only to the vehicle and no movement. This gives me a picture of the plane standing still relative to a sideline observer but lifting off then shooting forward. Does this sound right to you.
Mr. Revenge
Well, I picture it shooting forward then lifting off. But yeah, you are right. And one major difference between the skateboard ex. and the plane ex. is that the belt for the plane doesn't move until the plane starts moving, so you don't even have to worry about it overcoming a large amount of friction. So basically, it's even easier for the plane to move forward relative to the ground then the skateboard.

First, the plane overcomes static friction. Since before it does this the conveyor does nothing, it's no difference than if the plane were on a runway.

Second, since the plane is already moving, (and I suppose this happens instantly, but no matter) the belt starts moving backwards at the same speed of the plane, relative to the ground.

Third, since the belt can not apply a direct backwards force to the plane's body it has to rely on applying a force to the wheels of the plane. So now the wheels are moving extra fast. However, fast moving wheels doesn't really affect the plane too much (a little friction, nothing more.)

So the plane will feel a little resistance to it moving forward from the friction from it's fast moving wheels, but nothing that can come close to overcoming it's heavy duty engines.

QUOTE

"That would be energy expended but no movement of the board. The skate board would stay in place so this makes me think that the plane would stay in place if just enough engine power was applied to keep it there."

Exactly correct if the conveyor is already moving at a set rate. So yes, your thinking is correct. However, all it does is show that the energy required to hold the plane is place against a belt moving backwards at X mph is less than the energy required to move it forward at X mph on solid ground.

So when you are holding the skateboard in place (nice oiled wheels), it's nice and easy to do so even if the conveyor is moving very fast. However, it would be much, much harder to move the skateboard as fast as the conveyor along solid ground.

One last example:
Let's say the conveyor is moving at 100X speed, the exact speed it takes for our skateboard with wings to "take off". The skateboard is standing still on top of the belt, and you are holding it in place.

It might be a little hard to hold the skateboard in place, you might even have to hold it there wtih both hands. But it can be done, exp. if the wheels are well maintained and have as much time and money put into them as plane wheels (I'm sure plane wheels have to be top of the line). So you do have to image a skateboard with super wheels here.

Now, you could push the skateboard forward on the belt with your hands, even when it is moving so fast backwards. Thus, your skatebaord has exceeded 100X IN RELATION TO THE BELT. Not the ground yet, but that's ok.

Now think about moving the skateboard along the ground at 100X with only your hands. It would be impossible, or it would certainly require MUCH more force.

Since the skateboard would leave the conveyor at 100X, that means that if the conveyor behaved the same way as the conveyor with the plane (moving at the same speed the plane moved), then the TOTAL force exerted against the skateboard backwards would not exceed what it felt like when you were holding the skateboard in your hands with the belt moving at 100X. And for most of the process of the skateboard speeding up, the conveyor would be applying LESS friction than it does at 100X, making it even easier.

Back to a conveyor belt behaving the same way as in the plane example.
So if a belt moving at 100X applies 15F force in friction, then as long as the skateboard can overcome that 15F before it reaches a speed of 100X, it will be moving forward relative to the ground. But since you can imagine holding a skateboard in place against 15F not being too tough, then you can imagine that if you attached rocket engines to your skateboard, they would certainly be able to do a better job than your hand, and be able to push the skateboard forward even against the -100X speed of the belt.

So imagine a belt moving at 100X provides 15F force against the plane, and the plane moving along the ground at 100X has both 15F in force against it and 100F in drag(air resistance) against it.

To take off normally, the plane would require 115F in force to get to 100X in speed.
To take off on a belt, since the belt will never exceed applying another 15F in force, then as long as the plane can exceed somewhere around 130F (a little difference sinne I don't think both 15Fs will add together normally) in force if strained, it can take off on the belt.

I hope that helps and I didn't make it too confusing. I also have no idea about my numbers. But I can guarantee that a plane, given a slightly longer runway, could easily overcome a slight increase in resistance on it's takeoff.
sooks
sorry to jump in this a little late guys.... but i have been involved with this debate on a different forum. Yesterday i read the first 16 pages of this thread, and just came back now, i so i havent seen the last 7 pages but i thought i should give my input. j6p, your theory is somewhat right..... yes....in real life it would take some sort of force to keep the skateboard stationary, as it would take the plane some sort of force from the engine to maintain the same position IF THE BELT IS MOVING PRIOR TO ANY ACTION OF THE PLANE. This is only true due to the friction of the wheel bearings within the wheel. Now, if it were a frictionless wheel, then a moving conveyor belt wouldnt move the plane at all, just spiin the wheel. Now this is somewhat of a hard concept to imagine because there doesnt exist a frictionless item. in that case any force applied to the skateboard would propel it forward. with that being said....the only force going against of the plane would be the force of friction within the wheels which unlike a car are independent of the planes motion and do not push the plane forward. Since the only force acting against it is friction, i think the force of the thrust of the engine is great enough that it will overtake that.

Another point i feel that is important to make is that many people like to say that the conveyor is moving at the same speed as the plane and like to use an example starting with say the plane is moving x mph thus the conveyor is moving at x mph in the opposite direction. This is incorrect thinking in that it isnt movnig at the same speed as the plane, its moving at the same speed as the wheels which are free spinning. and you cant start an example saying it is moving so fast becuase it doesnt. In order for the conveyor to move, the plane must first move forward and there isnt any resistance to stop that movement and will thus continue to accelerate. And in order for the plane to be stationary CAN ONLY happen when it isnt moving....meaning that the conveyor WILL not be moving.
Mr. Revenge
I'm sorry I didn't completely follow the second half of your post there.
The original problem stated:

QUOTE
This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The conveyor IS moving at a speed opposite of the plane, not the wheels. The problem never mentions wheels.

So if the plane is moving at 5 mph, the conveyor HAS to be moving at -5 mph.
Simonetta
Haven't you guys seen air craft carriers? They use strong elastic bands to compliment the force of the jet so they can take off with less runway?

This would be the same of course the planes needs to propel itself plus the energy from the belt but in any case a conveyor belts would be too crude you need something like a kick off similar to the aircraft carrier.

Also there is no need for this they build airports on floating islands no need to complicated the most risky part of the flight take off and landing.
Guest
First with the aircraft carrier.... I could be wrong, but i dont recall ever seeing an aircraft carrier using strong elastic bands to launch a plane. Ive always seen a hydraulic cannon type thing that either pulled or pushed (cant remember off hand) to generate more speed quicker to use less runway. Then they use a steel cable run across the runway that a hook catches from the back of the plane to stop it.

Second, im sorry i didnt see the part about matching the plane speed. the example that i have read before is that the conveyor matches the wheel speed. Which i guess doesnt make much sense to me, its more logical that it would match the wheel speed. Either way, i dont think it makes much of a difference as the wheels are independent and the belt would have to match the force of the thrust in the opposite direction to prevent it from moving
Guest
hmmm.. You guys...
A car or any vehicle fitted with wings designed to lift the mass and enough thrust to propel it to a reasonable liftoff speed will fly. Just because the wheel based speedometer reads it is traveling at a certain speed, that is only generated by the moving conveyor. ANY movement in the conveyor implies forward motion of the vehicle.
So the vehicle WILL move forward, the speedometer will probably "pin" and the flying car will reach a speed sufficient to "liftoff".

remember....TECH...if you are still reading, the conveyor is tracking the vehicle's movement...not the other way around.
So. suppose the vehicle needs 50mph to lift off. The conveyor at that point will be going minus 50 mph and the speedometer on the winged car will be reporting 100 mph at liftoff.
Bloy
I might add to my last post above....I had to register to post again....
Although the vehicle will gain liftoff speed, as soon as this occurs, thrust will be lost(remember, this one is wheel driven and needs contact with the "runway") and it will settle back down, and probably blow the engine from over-revving when the wheels freespin for that second of liftoff. None the less, it will only REACH liftoff speed. Pray for a gust of headwind to make the FLYING experience last for a second or two longer.

OR, if there is a cliff at the end of the conveyor and the vehicle is propelled into free air, it then becomes a glider at stall speed which can be overcome by pointing the nose down, increasing speed. Also hope for a thermal to buy time before the inevitable "landing".

The only difference(well not the only difference) from this vehicle to one that relies on atmospheric propulsion(thrust) is that at liftoff speed, the "airplane" continues to accelerate and thus flies for an extended period of time.
Sage
QUOTE (Guest+Dec 2 2005, 07:48 AM)
hmmm.. You guys...
A car or any vehicle fitted with wings designed to lift the mass and enough thrust to propel it to a reasonable liftoff speed will fly. Just because the wheel based speedometer reads it is traveling at a certain speed, that is only generated by the moving conveyor. ANY movement in the conveyor implies forward motion of the vehicle.
So the vehicle WILL move forward, the speedometer will probably "pin" and the flying car will reach a speed sufficient to "liftoff".

remember....TECH...if you are still reading, the conveyor is tracking the vehicle's movement...not the other way around.
So. suppose the vehicle needs 50mph to lift off. The conveyor at that point will be going minus 50 mph and the speedometer on the winged car will be reporting 100 mph at liftoff.

Bloy, I was also tempted to bring this up, but these guys can't get to grips with the plane moving. How far do you think you'll get with a flying car!......I fear a new influx of (probably) sound physics applied with very poor logic and a total absence of common sense.
Chris in Sheffield
Winged Car.

If the conveyor is sensitive to the speed of the car relative to the conveyor, rather than the speed of the car relative to the ground, then the winged car won't take off, it will just be like a runner on a treadmill, and stay in one place relative to the ground and air. In fact, if the conveyor's mechanism (not it's surface, of course) was frictionless, and the conveyor was massless then the conveyor wouldn't need any power - the car would stay still and the driving wheels would just drive the conveyor backwards.

<<This is different from the case of the plane (which gets its propulsion not by pushing against the belt, but from a reaction force cuased by pushing the exhaust (jet) or propwash (propellor plane) backwards. The plane does take off (all the conveyor does is spin the plane's wheels at twice the normal speed).>>

However, the original problem stated that the conveyor is triggered by the vehicle's (then plane, now car) forward motion over the ground, rather than relative to the belt. But to my mind, this just says that the conveyor does whatever it has to do to keep the car stationary - which is exactly the same as above, "run backwards at a speed which matches the forward speed of the car's driving wheels". Car still doesn't take off !

If however, you insist that the belt ONLY moves when the car is moving forwards over the ground, but does so in such a way as to keep the car stationary, then the only possible steady state that meets both these conditions is that the car engine is stopped and so is the belt ! If you want add a third condtion that the car's drive wheels are turning, well tough ... NO steady state meets all these conditions, and what happens in the real world is down to the physics of the control mechanism.

... You can make it interesting by saying, what happens if the car is moving forward at max driving force (say 100mph) and THEN I turn on the belt .... ?
sooks
Chris is right........ Here is a note: The car along with a runner on a treadmill....or a bike on a treadmill are all different then the plane.... All those require friction acting between the ground and the wheels (or feet) to move forward. i.e. the cars engine turns a drivetrain which turns the wheels which PUSH against the ground to move it forward. Therefore if the ground in those cases is moving in the same speed backwards it wont go anywhere. The planes wheels do not propel the the plane. they are freespinning and simply act as a platform to the plane and to provide less friction. If the planes wheels were replaced with cinderblocks..it would still move. not as well as there is more friction...but it would move. its all about groundspeed and airspeed. The wheels will move at double the speed of the plane. say the plane is moving at 10 mph...conversely the belt is moving at 10 mph the opposite way, the wheels are then going 20mph. Here is a link that pretty much explains,...probably more effective then i can.

http://www.avweb.com/news/columns/191034-1.html

Also, the only way a car would work is if it was a jet car...like batmans.
j6p
I want to thank Mr.revenge and sooks for answering my post. I am still in the "it will fly" camp but I am beginning to sway again. Originally I was convinced that the bugger wouldn't fly then, reluctantly (pounding my desk) I started to see the light. Now I am becoming confused again but there is one experiment that will convince me that it either will or wont fly. I can't perform that experiment. I don't have a glider plane on wheels or a tread mill. If I had this equipment I could trash all these mind experiments and do the real thing. I would put that plane on the tread mill and put my finger in back of the plane and just hold it in place. I wouldn't push it forward, I wouldn't wind up the propeller. I would just keep it in place. I figure that the pressure I exerted to hold it in place would equal the force of the prop but there would be no air flow from a prop or jet. There would be energy exerted to keep the plane in place and the plane would exactly equal the speed of the tread mill.
Now if that plane takes to the air, I'd be totally convinced that that plane on the conveyor would fly. But if the small glider just sat there spinning wheels and not taking off then I would be equally convinced that the plane on the conveyor wouldn't take off. Not unless air flow or some other force were injected into the equation.
But I don't have this equipment. I do have a belt sander but I don't think it will go fast enough to validate the experiment.
Guest
QUOTE (j6p+Dec 2 2005, 02:57 PM)
I want to thank Mr.revenge and sooks for answering my post. I am still in the "it will fly" camp but I am beginning to sway again. Originally I was convinced that the bugger wouldn't fly then, reluctantly (pounding my desk) I started to see the light. Now I am becoming confused again but there is one experiment that will convince me that it either will or wont fly. I can't perform that experiment. I don't have a glider plane on wheels or a tread mill. If I had this equipment I could trash all these mind experiments and do the real thing. I would put that plane on the tread mill and put my finger in back of the plane and just hold it in place. I wouldn't push it forward, I wouldn't wind up the propeller. I would just keep it in place. I figure that the pressure I exerted to hold it in place would equal the force of the prop but there would be no air flow from a prop or jet. There would be energy exerted to keep the plane in place and the plane would exactly equal the speed of the tread mill.
Now if that plane takes to the air, I'd be totally convinced that that plane on the conveyor would fly. But if the small glider just sat there spinning wheels and not taking off then I would be equally convinced that the plane on the conveyor wouldn't take off. Not unless air flow or some other force were injected into the equation.
But I don't have this equipment. I do have a belt sander but I don't think it will go fast enough to validate the experiment.

Your experiment would prove nothing other than this: If the plane, powered by any type of engine you want, modulated the throttle at a setting low enough, it could just sit in one place while the belt moved. Of course, then you're back to violating the conditions in the problem...the belt only moves if the plane is moving. Not only that, there's plenty of thrust in reserve to start the plane moving forward.
Bloy
QUOTE (Chris in Sheffield+Dec 2 2005, 02:17 PM)
Winged Car.

If the conveyor is sensitive to the speed of the car relative to the conveyor, rather than the speed of the car relative to the ground, then the winged car won't take off, it will just be like a runner on a treadmill, and stay in one place relative to the ground and air.

If however, you insist that the belt ONLY moves when the car is moving forwards

For the sake of arguement.....

Chris,
I'm not sure if you are saying it will or will not fly, but....
if the conveyor is moving then the plane is moving...not standing still. if the plane is standing still, then the conveyor is not moving. How can the conveyor be traveling 50mph(example) and the plane NOT be moving. Yes the wheels of the plane are rotating, but

the PLANE must move to meet the criteria of the poser.

MOVING is NOT speed. SPEED is a way of expressing the movement.

So any talk of the plane standing still while the conveyor moves is only considering the wheels and not the entire plane.

sooks
j6p... you kind of have the right idea. but its a little faulty... that would only really work on a frictionless system... your not exerting any force to overcome the drag. there is a slight force of friction or drag. without any thrust, the plane will move backwards because of this (in your diagram) and simply putting your finger there will be only matching that force in the opposite direction. youd have to wind the propeller or push it from behind to be equating the thrust of the plane. In that link i sent,...one of his colleagues literally experimented that same way...but he actually used the propellers
sooks
QUOTE (Bloy+Dec 2 2005, 03:30 PM)
QUOTE (Chris in Sheffield+Dec 2 2005, 02:17 PM)
Winged Car.

If the conveyor is sensitive to the speed of the car relative to the conveyor, rather than the speed of the car relative to the ground, then the winged car won't take off, it will just be like a runner on a treadmill, and stay in one place relative to the ground and air.

If however, you insist that the belt ONLY moves when the car is moving forwards

For the sake of arguement.....

Chris,
I'm not sure if you are saying it will or will not fly, but....
if the conveyor is moving then the plane is moving...not standing still. if the plane is standing still, then the conveyor is not moving. How can the conveyor be traveling 50mph(example) and the plane NOT be moving. Yes the wheels of the plane are rotating, but

the PLANE must move to meet the criteria of the poser.

MOVING is NOT speed. SPEED is a way of expressing the movement.

So any talk of the plane standing still while the conveyor moves is only considering the wheels and not the entire plane.

I think kinda waht you are saying is there is a difference. Maybe the term that should start to be used would be velocity. where its speed and position. speed is purely relational.
Insyght
Car uses ground to push-against to move. When car on black ice, you can rev car and it will stand on the spot.

So put wings on car and speed down runway @ 300kph. You will get some lift (reason spoilers are handy for keeping cars pushed down to the ground).

Now, put the car on a belt, it will be like putting the car on ultra-slipperly black ice. Car will not move (Through air). It will be still, relative to the ground, but moving relative to the belt.

Car will not lift, because no movemented relative to ground is the same as no movement relative to the air.

Plane on other hand is not using ground to propel it's self. It is using air. It will push against the air regardless of what the belt is doing, so the plane will move forward, come off the belt (if it is short enough) or move along the belt (If it is long enough) and will continue to accelerate and take off.

Plane will move moving relative to ground, air and belt. The belt movement is just insignifcant as the forces it exerts is miniscule.

Will plane fly: Yes.
Will car fly: No.

Bloy
QUOTE (Insyght+Dec 2 2005, 04:23 PM)
Car uses ground to push-against to move. When car on black ice, you can rev car and it will stand on the spot.

So put wings on car and speed down runway @ 300kph. You will get some lift (reason spoilers are handy for keeping cars pushed down to the ground).

Now, put the car on a belt, it will be like putting the car on ultra-slipperly black ice. Car will not move (Through air). It will be still, relative to the ground, but moving relative to the belt.

Car will not lift, because no movemented relative to ground is the same as no movement relative to the air.

Plane on other hand is not using ground to propel it's self. It is using air. It will push against the air regardless of what the belt is doing, so the plane will move forward, come off the belt (if it is short enough) or move along the belt (If it is long enough) and will continue to accelerate and take off.

Plane will move moving relative to ground, air and belt. The belt movement is just insignifcant as the forces it exerts is miniscule.

Will plane fly: Yes.
Will car fly: No.

Will plane fly? Yes
Will car fly? no

But the car(winged) will not fly because it will only attain "liftoff speed" at which point friction with the "runway" is lost and there is no greater forward movement.
The wheels of the car will just be spinning at a higher rate of rotation so the MOVEMENT of the car matches the movement of the conveyor and thus satisfies the criteria of the posed question.

As stated, only a gust of headwind while at the required liftoff speed can make the car actually fly for a brief moment.
j6p
sooks, thanks again. I had to go back and read the original post and you are correct. The plane does move. I forgot this part of the post. I was just keeping the plane in place not pushing it forward. I am, again, fully ensconced in the "it will fly" camp. Now I can't see how it was so hard for me to visualize a plane moving forward and not flying.

QUOTE
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?
sooks
QUOTE (j6p+Dec 2 2005, 04:38 PM)
sooks, thanks again. I had to go back and read the original post and you are correct. The plane does move. I forgot this part of the post. I was just keeping the plane in place not pushing it forward. I am, again, fully ensconced in the "it will fly" camp. Now I can't see how it was so hard for me to visualize a plane moving forward and not flying.

QUOTE
A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?

Haha....yeah i was like that too right away...sometimes once you get so far into something, its hard to take a step back and look at the big picture.
Bloy
QUOTE (Bloy+Dec 2 2005, 04:36 PM)
QUOTE (Insyght+Dec 2 2005, 04:23 PM)
Car uses ground to push-against to move. When car on black ice, you can rev car and it will stand on the spot.

So put wings on car and speed down runway @ 300kph. You will get some lift (reason spoilers are handy for keeping cars pushed down to the ground).

Now, put the car on a belt, it will be like putting the car on ultra-slipperly black ice. Car will not move (Through air). It will be still, relative to the ground, but moving relative to the belt.

Car will not lift, because no movemented relative to ground is the same as no movement relative to the air.

Plane on other hand is not using ground to propel it's self. It is using air. It will push against the air regardless of what the belt is doing, so the plane will move forward, come off the belt (if it is short enough) or move along the belt (If it is long enough) and will continue to accelerate and take off.

Plane will move moving relative to ground, air and belt. The belt movement is just insignifcant as the forces it exerts is miniscule.

Will plane fly: Yes.
Will car fly: No.

Will plane fly? Yes
Will car fly? no

But the car(winged) will not fly because it will only attain "liftoff speed" at which point friction with the "runway" is lost and there is no greater forward movement.
The wheels of the car will just be spinning at a higher rate of rotation so the MOVEMENT of the car matches the movement of the conveyor and thus satisfies the criteria of the posed question.

As stated, only a gust of headwind while at the required liftoff speed can make the car actually fly for a brief moment.

someone tell me I'm wrong...and why
sooks
QUOTE (Bloy+Dec 2 2005, 04:54 PM)
QUOTE (Bloy+Dec 2 2005, 04:36 PM)
QUOTE (Insyght+Dec 2 2005, 04:23 PM)
Car uses ground to push-against to move. When car on black ice, you can rev car and it will stand on the spot.

So put wings on car and speed down runway @ 300kph. You will get some lift (reason spoilers are handy for keeping cars pushed down to the ground).

Now, put the car on a belt, it will be like putting the car on ultra-slipperly black ice. Car will not move (Through air). It will be still, relative to the ground, but moving relative to the belt.

Car will not lift, because no movemented relative to ground is the same as no movement relative to the air.

Plane on other hand is not using ground to propel it's self. It is using air. It will push against the air regardless of what the belt is doing, so the plane will move forward, come off the belt (if it is short enough) or move along the belt (If it is long enough) and will continue to accelerate and take off.

Plane will move moving relative to ground, air and belt. The belt movement is just insignifcant as the forces it exerts is miniscule.

Will plane fly: Yes.
Will car fly: No.

Will plane fly? Yes
Will car fly? no

But the car(winged) will not fly because it will only attain "liftoff speed" at which point friction with the "runway" is lost and there is no greater forward movement.
The wheels of the car will just be spinning at a higher rate of rotation so the MOVEMENT of the car matches the movement of the conveyor and thus satisfies the criteria of the posed question.

As stated, only a gust of headwind while at the required liftoff speed can make the car actually fly for a brief moment.

someone tell me I'm wrong...and why

You are wrong because the car will not achieve "lift off" speed. because the car will not move like i explained in one of my earlier posts. it does depend on the wheels and the wont be gaining on the ground. Its like a car on a dyno machine.
Guest
QUOTE (fmbga+Jul 19 2005, 07:43 PM)

Yes, the plane will take off, practically as it normally would. If you couldn't see the "runway" moving, you would see no difference to a normal takeoff, unless you could discern that the plane's wheels were spinning twice as fast as normal.

The reason: The plane, even if on the ground, accelerates relative to the AIR! Thanks to its wheels there is very little friction between it and the ground it stands on, so what the ground does is of almost no importance.

Now, if you would put wings on a car, that would be different. The takeoff would not work, because the car uses its wheels to push itself forward relative to the GROUND. It would not achieve any speed relative to the air, so its wings could create no lift.

ground propulsion vs air propulsion...
Bloy
no it is NOT like a car on a dyno machine! simply because on a dyno machine the "conveyor" is moving but the car is not. This violates the posed question in that the conveyor moves opposite the car MOVEMENT.
..in applying the question to the dyno machine, it will not operate unless the car also moves....NOT just the wheels.

Again and again...how can the conveyor move if the car is not moving?
Guest
Hi guys, I've been following this discussion for some time and I have to say I entirely agree with Insyght and Mech Tech - the plane won't fly!!

This is such BASIC physics but I can see a lot of you are going to take some convincing...

OK, look at it like this.

Say you've got a toy boat, and the boat's maximum speed is 10mph. Put the boat on a perfectly calm lake, and it whizzes around at 10mph. I think you'll also agree that the air is also flowing over the boat at 10mph. Right?

Now take the boat and find a river which is flowing *downstream* at 10mph. If you put the boat on the river facing in the opposite direction, how far does it move? It does not move at all. In fact, it's sitting *perfectly still* on the water. The forwards thrust of the boat is completely negated by the force of the river.

So how much air is flowing over the boat now?? None!!! It's stationary. The river might be moving, but the *air around the boat* is not moving at all! And this is what the problem is: the boat needs to be moving in relation to the *air around it* - NOT in relation to the river!!

Now put some wings on the boat. Increase the speed of the river by 10mph, and the speed of the boat-plane by the 10mph. How fast is it moving now?? It's still not moving!! If you were a person standing on the river bank watching this fiasco, the plane would still be sitting right in front of you, it would not have budged an inch. Even though it's now applying 20mph of *thrust*, the actual airspeed of the plane is still zero! And airspeed is what you need for a plane to fly...

In fact you'd even be able to walk up to the plane and feel with your own hands that there was NO air flowing over the wings WHATSOEVER!!

In order for this boat-plane to be able to take off, it would actually need to be travelling at 200% the speed of *whatever* the river is travelling at in order to get airborne. But since, in this example, the speed of the river (or the conveyor belt) *always* matches whatever the boat-plane is doing, your speed will never be more than 100%, it would never be able to move forwards from it's starting position, and the air would never flow over it.

It would not stand a chance in hell of taking off!

Have a nice day!
sooks
QUOTE (Bloy+Dec 2 2005, 05:15 PM)
no it is NOT like a car on a dyno machine! simply because on a dyno machine the "conveyor" is moving but the car is not. This violates the posed question in that the conveyor moves opposite the car MOVEMENT.
The dyno machine will not operate unless the car also moves....NOT just the wheels.

Again and again...how can the conveyor move if the car is not moving?

Yes...... but inorder for the car to move...it must transfer its movement THROUGH THE WHEELS..... once it does that..the conveyor moves the opposite direction thus it goes nowhere....
Bloy
QUOTE (Guest+Dec 2 2005, 05:16 PM)
Hi guys, I've been following this discussion for some time and I have to say I entirely agree with Insyght and Mech Tech - the plane won't fly!!

This is such BASIC physics but I can see a lot of you are going to take some convincing...

OK, look at it like this.

Say you've got a toy boat, and the boat's maximum speed is 10mph. Put the boat on a perfectly calm lake, and it whizzes around at 10mph. I think you'll also agree that the air is also flowing over the boat at 10mph. Right?

Now take the boat and find a river which is flowing *downstream* at 10mph. If you put the boat on the river facing in the opposite direction, how far does it move? It does not move at all. In fact, it's sitting *perfectly still* on the water. The forwards thrust of the boat is completely negated by the force of the river.

So how much air is flowing over the boat now?? None!!! It's stationary. The river might be moving, but the *air around the boat* is not moving at all! And this is what the problem is: the boat needs to be moving in relation to the *air around it* - NOT in relation to the river!!

Now put some wings on the boat. Increase the speed of the river by 10mph, and the speed of the boat-plane by the 10mph. How fast is it moving now?? It's still not moving!! If you were a person standing on the river bank watching this fiasco, the plane would still be sitting right in front of you, it would not have budged an inch. Even though it's now applying 20mph of *thrust*, the actual airspeed of the plane is still zero! And airspeed is what you need for a plane to fly...

In fact you'd even be able to walk up to the plane and feel with your own hands that there was NO air flowing over the wings WHATSOEVER!!

In order for this boat-plane to be able to take off, it would actually need to be travelling at 200% the speed of *whatever* the river is travelling at in order to get airborne. But since, in this example, the speed of the river (or the conveyor belt) *always* matches whatever the boat-plane is doing, your speed will never be more than 100%, it would never be able to move forwards from it's starting position, and the air would never flow over it.

It would not stand a chance in hell of taking off!

Have a nice day!

In order for YOUR example to meet the criteria of the question, the river must stop flowing. Once the boat is not moving upstream the river would have to defy gravity and stop flowing. Sure, the prop is still thrusting through the water, but the MOVEMENT of the boat is(in your example) is nil so therfore the water MOVEment must also be nil.
Bloy
QUOTE (sooks+Dec 2 2005, 05:17 PM)
QUOTE (Bloy+Dec 2 2005, 05:15 PM)
no it is NOT like a car on a dyno machine!  simply because on a dyno machine the "conveyor" is moving but the car is not.  This violates the posed question in that the conveyor moves opposite the car MOVEMENT.
The dyno machine will not operate unless the car also moves....NOT just the wheels.

Again and again...how can the conveyor move if the car is not moving?

Yes...... but inorder for the car to move...it must transfer its movement THROUGH THE WHEELS..... once it does that..the conveyor moves the opposite direction thus it goes nowhere....

But the conveyor is equalling the MOVEMENT of the car(winged)., not the movement of the wheels. So if the car is moving, the surface of the wheels will spin(MOVE) twice as fast as the conveyor to keep the MOVEMENT of the car equal to the MOVEMENT of the conveyor.
sooks
QUOTE (Guest+Dec 2 2005, 05:16 PM)
Hi guys, I've been following this discussion for some time and I have to say I entirely agree with Insyght and Mech Tech - the plane won't fly!!

This is such BASIC physics but I can see a lot of you are going to take some convincing...

OK, look at it like this.

Say you've got a toy boat, and the boat's maximum speed is 10mph. Put the boat on a perfectly calm lake, and it whizzes around at 10mph. I think you'll also agree that the air is also flowing over the boat at 10mph. Right?

Now take the boat and find a river which is flowing *downstream* at 10mph. If you put the boat on the river facing in the opposite direction, how far does it move? It does not move at all. In fact, it's sitting *perfectly still* on the water. The forwards thrust of the boat is completely negated by the force of the river.

So how much air is flowing over the boat now?? None!!! It's stationary. The river might be moving, but the *air around the boat* is not moving at all! And this is what the problem is: the boat needs to be moving in relation to the *air around it* - NOT in relation to the river!!

Now put some wings on the boat. Increase the speed of the river by 10mph, and the speed of the boat-plane by the 10mph. How fast is it moving now?? It's still not moving!! If you were a person standing on the river bank watching this fiasco, the plane would still be sitting right in front of you, it would not have budged an inch. Even though it's now applying 20mph of *thrust*, the actual airspeed of the plane is still zero! And airspeed is what you need for a plane to fly...

In fact you'd even be able to walk up to the plane and feel with your own hands that there was NO air flowing over the wings WHATSOEVER!!

In order for this boat-plane to be able to take off, it would actually need to be travelling at 200% the speed of *whatever* the river is travelling at in order to get airborne. But since, in this example, the speed of the river (or the conveyor belt) *always* matches whatever the boat-plane is doing, your speed will never be more than 100%, it would never be able to move forwards from it's starting position, and the air would never flow over it.

It would not stand a chance in hell of taking off!

Have a nice day!

Your right on a few facts....It IS BASIC PHYSICS..... your first aspect you wrong with your toy boat becase its not the same...the boat is propelling itself with the engine in the water through a prop...it would be different if your talking about a fan boat.... a fan boat will move mostly ragardless of the water current (only due to some friction) a fan boat is equivalent to the plane because it is being pushed from behind from the fan forcnig air...the water is negligible..much like are the wheels/belt negligible in the plane question.

You are somehwat right on 200%....cause that is what the wheels would be moving in comparison to waht they normally are....or to the relative planes velocity.

so yes..it would stand a pretty damn good chance in hell of taking off....(if you negate that the wheels could withstand the extra speed) .... if this is still confusing you....please look at the link i sent on the previous page of this thread...i think they do a better job of explaining it then i do.
sooks
QUOTE (Bloy+Dec 2 2005, 05:30 PM)
QUOTE (sooks+Dec 2 2005, 05:17 PM)
QUOTE (Bloy+Dec 2 2005, 05:15 PM)
no it is NOT like a car on a dyno machine!  simply because on a dyno machine the "conveyor" is moving but the car is not.  This violates the posed question in that the conveyor moves opposite the car MOVEMENT.
The dyno machine will not operate unless the car also moves....NOT just the wheels.

Again and again...how can the conveyor move if the car is not moving?

Yes...... but inorder for the car to move...it must transfer its movement THROUGH THE WHEELS..... once it does that..the conveyor moves the opposite direction thus it goes nowhere....

But the conveyor is equalling the MOVEMENT of the car(winged)., not the movement of the wheels. So if the car is moving, the surface of the wheels will spin(MOVE) twice as fast as the conveyor to keep the MOVEMENT of the car equal to the MOVEMENT of the conveyor.

ok bloy...i get what your saying now..... however, then it is a poorly worded question. it either has to match the wheels or it wouldnt work becuase it cant move the wheels twice as fast on the car without breaking something.... the wheels are attached to the drivetrain.....moving the wheels twice as fast moves the car twice as fast.

but actuawlly thinking about it as i was writing this....regardless if it is matching wheel speed or car speed...its the same...because the wheels moving the car, the car can only go as fast as the wheels...they are what propelling it...(unless there is a loss of friction...i.e. wheel spin (like on ice)) thus again...i say it would be matching it and not go anywhere..
Bloy
QUOTE (sooks+Dec 2 2005, 05:36 PM)
QUOTE (Bloy+Dec 2 2005, 05:30 PM)
QUOTE (sooks+Dec 2 2005, 05:17 PM)
QUOTE (Bloy+Dec 2 2005, 05:15 PM)
no it is NOT like a car on a dyno machine!  simply because on a dyno machine the "conveyor" is moving but the car is not.  This violates the posed question in that the conveyor moves opposite the car MOVEMENT.
The dyno machine will not operate unless the car also moves....NOT just the wheels.

Again and again...how can the conveyor move if the car is not moving?

Yes...... but inorder for the car to move...it must transfer its movement THROUGH THE WHEELS..... once it does that..the conveyor moves the opposite direction thus it goes nowhere....

But the conveyor is equalling the MOVEMENT of the car(winged)., not the movement of the wheels. So if the car is moving, the surface of the wheels will spin(MOVE) twice as fast as the conveyor to keep the MOVEMENT of the car equal to the MOVEMENT of the conveyor.

ok bloy...i get what your saying now..... however, then it is a poorly worded question. it either has to match the wheels or it wouldnt work becuase it cant move the wheels twice as fast on the car without breaking something.... the wheels are attached to the drivetrain.....moving the wheels twice as fast moves the car twice as fast.

but actuawlly thinking about it as i was writing this....regardless if it is matching wheel speed or car speed...its the same...because the wheels moving the car, the car can only go as fast as the wheels...they are what propelling it...(unless there is a loss of friction...i.e. wheel spin (like on ice)) thus again...i say it would be matching it and not go anywhere..

okay,
If the conveyor is moving 50MPH, and the car is moving 50MPH in the opposite direction, What would the speedometer(which is translating from the RPMS of the wheels of the car) be showing? ...right 100MPH . This is still in the range of most vehicles to achieve. Now, given that the liftoff speed of the car/plane is designed for 50MPH, is not the MOVEMENT of the car just ready to fly at 50MPH?
Sage
Bloy, what did I say;-)

I'm sure that some of you non-flyers are just having a laugh......

You know those moving walkways at the airport? Imagine one 1 mile long (from A to and belt moving at 4mph, if you stood on it and remained still it would cart you from A to B in 15minutes. But you're a cheeky little monkey and decide to walk along it the wrong way (from B to A). As soon as you step on, your little legs need to do a pretty brisk walk just to remain stationary. In order to actually MOVE you need to walk faster than the speed of the belt. Now, according to the puzzle the belt moves at the same speed as you do (but in opposite direction). Therefore you are moving at 4mph from B to A - it will still take you 15 minutes to travel the length on the concourse. But you will (if you're anything like me) but sweaty and knackered from making your legs run at 8mph to achieve this feat.

Planes, cars, pigs, hippopotami whatever you choose - strap a wing on them and they will fly!!!
chrisinsheffield
There are three different ideas being discussed here (as well as the car/plane options). It depends how you have programmed the conveyor's speed controller. The fourth option is the one originally asked !

I assume for arguments sake that the vehicle does its best to take off, and that the top speed for both vee-hickles is 100kph (airspeed for plane, wheelspeed for car) and that the plane and car take off when their airspeed reaches 50kph. I assume that conveyor's max speed is 200kph. I also assume that there is no wind, so groudspeed = airspeed, where groundspeed means relative to ground, NOT relative to conveyor (which is wheelspeed).

I also assume that the difference between a winged car and a plane is that the plane gets its thrust from its jet or prop (very roughly speaking "against the air"), not from its undercarrriage wheels, but the car gets its thrust from spinning its wheels. This means that if the winged car took off, it would then only be a glider.

1) "Increase conveyor speed (up to max) until vehicle has zero groundspeed, then maintain this speed."

- Car ends up with zero airspeed/groundspeed. Conveyor going backwards at 100kph. Wheelspeed 100kph. Car does not take off.

- Plane reaches 50kph airspeed/groundspeed. Conveyor reaches 200kph (max speed) in attempt to catch up with wheelspeed. Wheelspeed ends up at 250kph. Plane takes off.

2) "Spin conveyor backwards to match vehicle's wheelspeed (forwards)"

Car - Results as above

Plane - results as above.

3) "Spin conveyor at same speed (backwards) that vehicle is travelling (forwards) relative to ground"

- Car ends up at 50kph airspeed/groundspeed. Conveyor going at 50kph. Wheelspeed 100kph. Car takes off (just!, it is at max wheelspeed).

- Plane reaches 50kph groundspeed/airspeed. Conveyor going at 50kph. Wheelspeed 100kph. Plane takes off.

4) Increase conveyorspeed until vehicle has zero groundspeed, then stop conveyor. Repeat as required. (People are sayig that this is the one that was originally asked).

- Plane as 1

- Car. Depends entirely on the real-life behaviour of the real-life conveyor such as its accelleration, stretchinness, etc, on the wind resistance of the car, the skiddability of the tyres, etc. Will tend to oscillate wildly between two states (stopped, and leaping forwards, stopped, and leaping forwards).
Sage
Sorry about misplaced smiley - obviously should just read A to B.
Bloy
Let's put it this way... without adding any elements to the posed question.

will the plane reach liftoff speed? Yes
will the winged car/plane reach liftoff speed? yes
SkiSTX
Car vs. Plane

SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

-PLANE -> Engines throttle up. Planes speed relative to EARTH = 50mph. Planes airspeed is 50mph and if it had a spedometer on it's wheels, it would read 100mph. Conveyor is moving at 50mph because the Plane's speed relative to earth is 50mph. Plane acheives take off speed.

SCENARIO 2 = you read the question to mean speed of object according to it's own spedometer. (a plane measures airspeed while a car measures wheel spin).

-CAR -> Car steps on accelerator. Car's speed according to speedomer = 50mph. It measures the spin of the tires. Conveyor is moving at 50mph because the car's spedometer says 50mph. 50 minus 50 = zero. Car remains stationary and with no wind passing by and does not take off.

-PLANE -> Engines throttle up. Plane's speed according to it's speedometer = 50mph. It measures the air rushing by and ignores the spin of the tires. Conveyor is moving at 50mph because the Plane's air speed is 50mph. Plane acheives take off speed.

SCENARIO 3 = you read the question to mean speed of object RELATIVE to the CONVEYOR.

-CAR -> Car steps on accelerator. It doesn't matter how fast the engine or wheels turn, the conveyor also turns and as long as the car remains stationary the speed of the conveyor will equal the speed of the Car along the conveyor and conditions of the question are met. With no wind blowing past, the engine, wheels and conveyor can go any speed and no one will get anywhere.

-PLANE -> Engines throttle up. It doesn't matter how hard the jets or props push, the conveyor also turns and as long as the Plane remains stationary, the speed of the conveyor will equal the speed of the plane along the conveyor and conditions of the question are met. Because the Planes engines push against the calm air, the conveyor would have to accelerate to enourmous speeds and the Planes wheels would fall off and everyone would die. The result of this is that the plane will not take off.

I read the question literally. When it says the plane or car or whatever "moves forward", I take that to mean really and truely move forward through space, through the air, if someone were sitting in a lawn chair watching this crazy experiment they would have to turn their head to watch it go by. Scenario 1 is the only realistic way to read this question in my opinion.

So... HOW DO YOU READ THE QUESTION???
Bloy
QUOTE (SkiSTX+Dec 2 2005, 06:07 PM)
Car vs. Plane

SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator.  Car's speed relative to EARTH = 50mph.  Car's spedometer = 100mph because it measures the spin of the tires.  Conveyor is moving at 50mph because the car's speed relative to earth is 50mph.  Car acheives take off speed through the air.

-PLANE -> Engines throttle up.  Planes speed relative to EARTH = 50mph.  Planes airspeed is 50mph and if it had a spedometer on it's wheels, it would read 100mph.  Conveyor is moving at 50mph because the Plane's speed relative to earth is 50mph.  Plane acheives take off speed.

SCENARIO 2 = you read the question to mean speed of object according to it's own spedometer.  (a plane measures airspeed while a car measures wheel spin).

-CAR -> Car steps on accelerator.  Car's speed according to speedomer = 50mph.  It measures the spin of the tires.  Conveyor is moving at 50mph because the car's spedometer says 50mph.  50 minus 50 = zero.  Car remains stationary and with no wind passing by and does not take off.

-PLANE -> Engines throttle up.  Plane's speed according to it's speedometer = 50mph.  It measures the air rushing by and ignores the spin of the tires.  Conveyor is moving at 50mph because the Plane's air speed is 50mph.  Plane acheives take off speed.

SCENARIO 3 = you read the question to mean speed of object RELATIVE to the CONVEYOR.

-CAR -> Car steps on accelerator.  It doesn't matter how fast the engine or wheels turn, the conveyor also turns and as long as the car remains stationary the speed of the conveyor will equal the speed of the Car along the conveyor and conditions of the question are met. With no wind blowing past, the engine, wheels and conveyor can go any speed and no one will get anywhere.

-PLANE -> Engines throttle up.  It doesn't matter how hard the jets or props push, the conveyor also turns and as long as the Plane remains stationary, the speed of the conveyor will equal the speed of the plane along the conveyor and conditions of the question are met.  Because the Planes engines push against the calm air, the conveyor would have to accelerate to enourmous speeds and the Planes wheels would fall off and everyone would die.  The result of this is that the plane will not take off.

I read the question literally.  When it says the plane or car or whatever "moves forward", I take that to mean really and truely move forward through space, through the air, if someone were sitting in a lawn chair watching this crazy experiment they would have to turn their head to watch it go by.  Scenario 1 is the only realistic way to read this question in my opinion.

So... HOW DO YOU READ THE QUESTION???

whew..... nicely put!
Insyght
QUOTE
SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

This is so wrong (Sorry).

car spedometer = car speed.

If you have 100 on the clock, then the car is moving (Or could be moving) 100 mph. The belt would detect this and move at 100mph in opposite direction.

So relative to ground (and air) you have:

(car speed 100mph) - (belt speed 100mph) = 0

relative to the belt, you have:

(car speed 100mph) + (belt speed 100mph) = 200 mph

Therefore car does NOT take off...

Oh no... here we go again...
Bloy
Sorry Insyght,
speedometer does NOT indicate car speed...
What happens when you put bigger tires on the wheels....the speedometer is not indicating the car speed as calibrated with other tires. Speedometer only indicates wheel motion, not vehicle motion....

Heh..winter driving should tell you that! I've achied well over 50MPH while having little forward motion...er MOVEMENT.
SkiSTX
QUOTE (Insyght+Dec 2 2005, 06:22 PM)
QUOTE
SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

This is so wrong (Sorry).

car spedometer = car speed.

But speedometer is not telling you speed relative to the EARTH which is proposed in scenario ONE. I think you are reading the question as scenario TWO.

And no need to be sorry... I think this is all just semantics anyway!

Are you more in tune with Scenario 2?
Guest
QUOTE (Insyght+Dec 2 2005, 06:22 PM)
QUOTE
SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

This is so wrong (Sorry).

car spedometer = car speed.

If you have 100 on the clock, then the car is moving (Or could be moving) 100 mph. The belt would detect this and move at 100mph in opposite direction.

So relative to ground (and air) you have:

(car speed 100mph) - (belt speed 100mph) = 0

relative to the belt, you have:

(car speed 100mph) + (belt speed 100mph) = 200 mph

Therefore car does NOT take off...

Oh no... here we go again...

skis..that was perfectly put, except for part one like insyght said.

i think bloy understood it, just had a differnt viewpoint

insyght....good description. except for the very first part where you say 100 on the clock maens your moving 100. tahts not true...if your on ice and you punch it and the wheels are sliding, the speedo will rev way high. and wait...i just saw your part saying or could be moving...so you can pretty much disrgard this.. haha.

Just remember that speed is relative. velocity isnt. as to say velocity describes its relevence
sooks
QUOTE (Insyght+Dec 2 2005, 06:22 PM)
QUOTE
SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

This is so wrong (Sorry).

car spedometer = car speed.

If you have 100 on the clock, then the car is moving (Or could be moving) 100 mph. The belt would detect this and move at 100mph in opposite direction.

So relative to ground (and air) you have:

(car speed 100mph) - (belt speed 100mph) = 0

relative to the belt, you have:

(car speed 100mph) + (belt speed 100mph) = 200 mph

Therefore car does NOT take off...

Oh no... here we go again...

skis..that was perfectly put, except for part one like insyght said.

i think bloy understood it, just had a differnt viewpoint

insyght....good description. except for the very first part where you say 100 on the clock maens your moving 100. tahts not true...if your on ice and you punch it and the wheels are sliding, the speedo will rev way high. and wait...i just saw your part saying or could be moving...so you can pretty much disrgard this.. haha.

Just remember that speed is relative. velocity isnt. as to say velocity describes its relevence
sooks
QUOTE (SkiSTX+Dec 2 2005, 06:34 PM)
QUOTE (Insyght+Dec 2 2005, 06:22 PM)
QUOTE
SCENARIO 1 = you read the question to mean speed of object RELATIVE to the EARTH

-CAR -> Car steps on accelerator. Car's speed relative to EARTH = 50mph. Car's spedometer = 100mph because it measures the spin of the tires. Conveyor is moving at 50mph because the car's speed relative to earth is 50mph. Car acheives take off speed through the air.

This is so wrong (Sorry).

car spedometer = car speed.

But speedometer is not telling you speed relative to the EARTH which is proposed in scenario ONE. I think you are reading the question as scenario TWO.

And no need to be sorry... I think this is all just semantics anyway!

Are you more in tune with Scenario 2?

Skis, i think you answered your own statement there. You said the speedo isnt giving your speed relative to earth. Therefore insyght is right. your statement for scenario 1 says cars speed relative to earth...its not moving so its 0. you meant to say cars wheel speed compared to earth.
Bloy
QUOTE (Bloy+Dec 2 2005, 06:28 PM)
Sorry Insyght,
speedometer does NOT indicate car speed...
What happens when you put bigger tires on the wheels....the speedometer is not indicating the car speed as calibrated with other tires. Speedometer only indicates wheel motion, not vehicle motion....

Heh..winter driving should tell you that! I've achied well over 50MPH while having little forward motion...er MOVEMENT.

..so I was fooling myself if I thought I was actually MOVING the speedometer indication of 50MPH
SkiSTX
I think I do mean velocity instead of speed... now I'M confused!

But if a car is on a conveyor that is going at a speed/velocity of 50mph, and the cars speedometer/wheels spin says 100mph, assuming you are in a convertable, the wind will be blowing in your face at 50mph.... I think this is what I meant to say.
Insyght
QUOTE
Sorry Insyght,
speedometer does NOT indicate car speed...
What happens when you put bigger tires on the wheels....the speedometer is not indicating the car speed as calibrated with other tires. Speedometer only indicates wheel motion, not vehicle motion....but most speedometers with give you SOME accuracy.

Your tire size should always match you manufacturers specs. If not, then you can be reading 100 in the car, but be doing 110 outside + will be breaking the law in some places.

The assumption here is a legally calibrated vehicle.
Bloy
QUOTE (Insyght+Dec 2 2005, 06:52 PM)
QUOTE
Sorry Insyght,
speedometer does NOT indicate car speed...
What happens when you put bigger tires on the wheels....the speedometer is not indicating the car speed as calibrated with other tires. Speedometer only indicates wheel motion, not vehicle motion....but most speedometers with give you SOME accuracy.

Your tire size should always match you manufacturers specs. If not, then you can be reading 100 in the car, but be doing 110 outside + will be breaking the law in some places.

The assumption here is a legally calibrated vehicle.

so then you're saying that the conveyor is tracking the speedometer and NOT the Movement of the vehicle?
Insyght
QUOTE
But if a car is on a conveyor that is going at a speed/velocity of 50mph, and the cars speedometer/wheels spin says 100mph, assuming you are in a convertable, the wind will be blowing in your face at 50mph.... I think this is what I meant to say.

Yep. If the belt is locked down to 50 and you are driving at 100, then for every 1 step back you are making 2 steps forward. You would have a wind of 50 hitting you in the face.

However... according to the riddle (LOL) the belt speed matches the car speed. So this should never happen
Insyght
QUOTE
o then you're saying that the conveyor is tracking the speedometer and NOT the Movement of the vehicle?

Nope. The speedo is a guess-timate of the car speed. Based on known wheel circumference and known axle turn speed.

The belt would not see the speedo, only the physical force exerted by the wheel on the belt. Therefore the belt would track the true speed of the car, unlike the speedo.
SkiSTX
QUOTE (Insyght+Dec 2 2005, 06:56 PM)
QUOTE
But if a car is on a conveyor that is going at a speed/velocity of 50mph, and the cars speedometer/wheels spin says 100mph, assuming you are in a convertable, the wind will be blowing in your face at 50mph.... I think this is what I meant to say.

Yep. If the belt is locked down to 50 and you are driving at 100, then for every 1 step back you are making 2 steps forward. You would have a wind of 50 hitting you in the face.

However... according to the riddle (LOL) the belt speed matches the car speed. So this should never happen

Right, the belt speed matches the car speed.. which is what the riddle states so it WILL happen...

The car's speed( or velocity or whatever) is 50 and the conveyor's is 50 - which equal and is what the riddle states. Just because the speedometer says 100 doesn't mean the car is actually travelling 100. It is travelling at 50.. becasue, remember, in scenario ONE, we are measuring all things relative to the unmoving EARTH.

If a car is on black ice and their speedometer says 100mph but the cop that is sitting in his radar trap guns the car going by at 10 mph, can the cop give him a ticket.

(The answer is yes, for reckless driving by spinning the tires out of control, but NOT FOR SPEEDING!)

But as far as the plane goes, it measures air speed, and the wheels' speed is irrelevant.
Bloy
QUOTE (Insyght+Dec 2 2005, 06:56 PM)
QUOTE
But if a car is on a conveyor that is going at a speed/velocity of 50mph, and the cars speedometer/wheels spin says 100mph, assuming you are in a convertable, the wind will be blowing in your face at 50mph.... I think this is what I meant to say.

Yep. If the belt is locked down to 50 and you are driving at 100, then for every 1 step back you are making 2 steps forward. You would have a wind of 50 hitting you in the face.

However... according to the riddle (LOL) the belt speed matches the car speed. So this should never happen

..that is.... you are driving at 100 according to the speedo. but you are actually traveling at 50.. If the wheel driven plane's takeoff speed is 50, then it is there!
Bloy
I have this thread listed under my "frivolous entertainment" section.
Sage
c'mon guys. The puzzle is so much simpler than all this. It states 'The plane moves'. What is unclear about that?

We need to know one thing to solve this. Does the plane move? Yes, it does, it's written right there in the question. The rest becomes irrelevant.

Back to my travellator at the airport. Drop something on the walkway at A and 15 minutes later it will be at B, travelling at 4mph. And we know that if the conveyor is moving at 4mph, A to B, then you are moving at 4mph B to A and so it takes you 15 minutes to go from B to A. And to achieve this you need to run at 8 mph! How difficult is that to understand. The conveyor does not match the speed of the wheels or the speed at which you walk. It mirrors your progress ie how fast you move from one place to the next.

It is the only solution that actually fits all parts of the puzzle. It is only open to debate if you do not understand (or, more likely, refuse to understand) the English language.

Now, I'm going to try extra hard not to be drawn back into this.
Guest
http://videos.streetfire.net/player.aspx?f...8D-D6BA1A43A06B

^^

I have to say that this video which was posted earlier, is a woefully inadequate attempt at proving anything whatsoever!!!

Does it show how fast the skateboard is actually moving in mph? Does the guy adjust the speed of the 'runway' in order to match it the way the conveyor's computer would? Does it show how much thrust the fan is producing? I mean, this is just a laughably lame experiment!

In fact it proves nothing whatsoever, because it doesn't even consider the power to weight ratio between the fan and the skateboard! The board itself is incredibly light compared to how much a jumbo would weigh. And the fan is out of proportion to the board and producing no thrust whatsoever.

The only reason the skateboard moves is because the guy's pulling the paper out from under it. Now try doing that with a jumbo on a conveyor. The jumbo would not move forwards, and you could not 'pull the runway back' from underneath it. The plane would only move backwards on the conveyor if you tried!

What you're forgetting is the effect of gravity holding the plane down on the conveyor. Gravity needs to be overcome if the plane is going to fly.

But every inch of momentum the plane tries to gain going forwards, it would lose again due to the conveyor moving backwards at an equal speed. There is no 'free rolling' of the wheels as some people have suggested. For this to happen, the plane itself would have to be weightless.

You're talking about an aircraft which weighs hundreds of tonnes, all of which is supported on these tiny little wheels, and which is the plane's only point of contact with the conveyor.

Insyght
The plane has been solved long ago... then someone included a CAR with wings (LOL) Which is different because of no-air thrust, so movement forward would not be possible...

Yet some feel as though the car would move forward, even though the riddle explicitly states that the belt matches car speed.

Look the riddle cannot be used on a CAR with wings?

Why? these two are mutually exclusive:

-> Belt matches speed of car
-> car moves forward.

Both are not possible with the car. Only one is.

If you say belt matches speed, then car cannot move.
If you say car moves, then belt cannot match speed.
sooks
QUOTE (Insyght+Dec 2 2005, 09:03 PM)
The plane has been solved long ago... then someone included a CAR with wings (LOL) Which is different because of no-air thrust, so movement forward would not be possible...

Yet some feel as though the car would move forward, even though the riddle explicitly states that the belt matches car speed.

Look the riddle cannot be used on a CAR with wings?

Why? these two are mutually exclusive:

-> Belt matches speed of car
-> car moves forward.

Both are not possible with the car. Only one is.

If you say belt matches speed, then car cannot move.
If you say car moves, then belt cannot match speed.

thank you insyght...very well put.

yes the skateboard demo isnt perfect....but its pretty damn good example. if you look...he didnt start pulling the paper to get the skateboard moving...and the thrust doesnt have to overcome gravity...gravity is only countered by the normal force straight vertical.... the only force it must overcome is the frictional force, mostly from the wheel bearings to move horizontally. and how much thrust is almost irrelevant... relatively speaking..it doesnt take a whole lot to move the plane. ive seen people move the plane by them selves by pulling a rope and thats not even pulling directly sideways. granted thats barely moving the plane but the output of a man compard to what a 100,000hp jet engine is a big difference.... but basically the wheel and belt are negligible. Why am i getting into this again. i really cant see how people keep arguing aganist this...go to my post where i put up a link a page or 2 ago.... it explains it.
Sage
QUOTE (Insyght+Dec 2 2005, 09:03 PM)
the riddle explicitly states that the belt matches car speed.

So it does. And speed is defined as the distance travelled over a period of time. If the car does not move neither does the belt (wheels rotating does not constitute the car moving). If the car moves 1 metre in 1 sec so does the belt which means that the wheels must rotate a distance of 2m. Hence the relative speed is 2m/s, your speedo will read 2m/s, your car will 'think' it is doing 2m/s, but it will still only have moved forward 1m in 1 sec - the same as the belt.

As I've said too many times now - the puzzle is pure logic, it works for any vehicle.
Insyght
Grammer again?

If you put your car on a dyno and rev it till the speedo says 100mph, what speed would you say your car is traveling?

Looks like you are saying 0mph, because the car is on the dyno...

Look at the problem with you definition of "speed";

1. Car at 0 meters per sec (mps) wheel speed. Belt @ 0 mps. 0 mps movement physically forward.

2. Car at 10 mps wheel speed. Car will try and move forward physically 10mps. Belt must spin backwards @ car Speed (the physically moving forward speed). This results in all of the cars 10mps forward movement being sucked up in 10mps backwards speed of the belt, leaving as a result 10mps wheel speed, 10mps backwards belt speed and 0mps physical speed.

3. Car at 50 mps wheel speed. Car try to move forward 50mps, cant, cos belt must match forward speed, so you end up with wheel speed 50 mps, belt speed back 50 mps and physically moving forward speed 0mps.

Tell me. As what speed would have to travel for this logic to break and forward movement starting to occur?
Guest_Jim
Back to the Airplane for a minute . . .

if the conveyor belt created sufficient headwind for the aircraft (on account of the friction between the conveyor belt and the air) then the aircraft could actually take off faster than otherwise (Assuming frictionless wheels and an aircraft mounted engine).

Sage
QUOTE (Insyght+Dec 2 2005, 10:34 PM)
Grammer again?

If you put your car on a dyno and rev it till the speedo says 100mph, what speed would you say your car is traveling?

Looks like you are saying 0mph, because the car is on the dyno...

Look at the problem with you definition of "speed";

1. Car at 0 meters per sec (mps) wheel speed. Belt @ 0 mps. 0 mps movement physically forward.

2. Car at 10 mps wheel speed. Car will try and move forward physically 10mps. Belt must spin backwards @ car Speed (the physically moving forward speed). This results in all of the cars 10mps forward movement being sucked up in 10mps backwards speed of the belt, leaving as a result 10mps wheel speed, 10mps backwards belt speed and 0mps physical speed.

3. Car at 50 mps wheel speed. Car try to move forward 50mps, cant, cos belt must match forward speed, so you end up with wheel speed 50 mps, belt speed back 50 mps and physically moving forward speed 0mps.

Tell me. As what speed would have to travel for this logic to break and forward movement starting to occur?

You mean semantics not grammar but that's another debate.

I agree with all the examples you illustrate. But none of these describe the puzzle.

It's not my definition of speed. Find a textbook, Speed = Distance/Time. With no distance travelled the car has no speed no matter how fast you make the wheels rotate. With no distance travelled the car has not moved. The puzzle tells us the car (sorry, plane) moves.

Hold your hands out in front of you and point with each forefinger. Now put the tip of each forefinger on the table so that they are right next to each other. Move your right finger 6 inches to the right whilst simultaneously moving your left finger 6 inches to the left. They are now 12 inches apart, each finger having moved 6 inches over the same period of time. This exactly describes the motion of our plane/car and the belt in this puzzle. They each move (ie travel a fixed distance)at the same rate in opposite directions at the same time. How much thrust you need to apply, how fast you have to rev the engine, how quickly the wheels need to spin, is irrelevant the solving the puzzle.
sooks
QUOTE (Sage+Dec 2 2005, 11:13 PM)
QUOTE (Insyght+Dec 2 2005, 10:34 PM)
Grammer again?

If you put your car on a dyno and rev it till the speedo says 100mph, what speed would you say your car is traveling?

Looks like you are saying 0mph, because the car is on the dyno...

Look at the problem with you definition of "speed";

1. Car at 0 meters per sec (mps) wheel speed. Belt @ 0 mps. 0 mps movement physically forward.

2. Car at 10 mps wheel speed. Car will try and move forward physically 10mps. Belt must spin backwards @ car Speed (the physically moving forward speed). This results in all of the cars 10mps forward movement being sucked up in 10mps backwards speed of the belt, leaving as a result 10mps wheel speed, 10mps backwards belt speed and 0mps physical speed.

3. Car at 50  mps wheel speed. Car try to move forward 50mps, cant, cos belt must match forward speed, so you end up with wheel speed 50 mps, belt speed back 50 mps and physically moving forward speed 0mps.

Tell me. As what speed would have to travel for this logic to break and forward movement starting to occur?

You mean semantics not grammar but that's another debate.

I agree with all the examples you illustrate. But none of these describe the puzzle.

It's not my definition of speed. Find a textbook, Speed = Distance/Time. With no distance travelled the car has no speed no matter how fast you make the wheels rotate. With no distance travelled the car has not moved. The puzzle tells us the car (sorry, plane) moves.

Hold your hands out in front of you and point with each forefinger. Now put the tip of each forefinger on the table so that they are right next to each other. Move your right finger 6 inches to the right whilst simultaneously moving your left finger 6 inches to the left. They are now 12 inches apart, each finger having moved 6 inches over the same period of time. This exactly describes the motion of our plane/car and the belt in this puzzle. They each move (ie travel a fixed distance)at the same rate in opposite directions at the same time. How much thrust you need to apply, how fast you have to rev the engine, how quickly the wheels need to spin, is irrelevant the solving the puzzle.

what your explaining works for the plane..but does not work for the car. the car does not move... it will stay in place no matter how much you rev the engine. a perfect example is the moving walkways at an airport. if you walked on them backwards you have to wlak as fast as teh walkway to stay in the same place. if you walk slower than the belt youll move backwards...if you walk faster than the walkway youll move forward but at your walking speed - the walkway speed.
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