mtoom

Are B and E both correct answers to this question?

------------------------------------------------------
QUESTION:

If the net force on an object ceases, then the object __________.

A. decelerates
B. coasts at constant speed
C. accelerates
D. doesn’t obey Newton’s laws anymore
E. has zero speed
------------------------------------------------------

Do you agree with the claim that E is correct because: an object is moving in space along a straight line in space. It is slowing down. If the net force on the object ceases at the instant that the velocity equals zero, then E must also be correct.
rpenner
You misunderstand if and then.

if Proposition α, then Proposition β means:

It is never the case that Proposition β is false and Proposition α is true.

Another way to say that is Proposition α implies Proposition β.
Another way to say that is Proposition α is a sufficient condition for Proposition β.

If it is daytime, then
a. the sun will be above the horizon
b. the children will eat library paste
c. you will be allowed to graduate.

Even if you have a photo of children at their graduation ceremony eating paste, the only thing that it being daytime proves is that the sun (even if we can't see it) is above the local horizon.

In fact for that case the two definitions are logically equivalent.
Proposition α is logically equivalent to Proposition β.
Another way to say that is if and only if Proposition α then Proposition β.

One way to set up a logical equivalence is by definition.

By definition, when the sun is above the horizon, we call that daytime.

By Newton's second law, net force is defined as mass times the time derivative of velocity.
By the laws of Newtonian physics, all bodies have a non-zero mass.
By the laws of algebra of real numbers, if a product is zero, then at least one of its factors is zero.
By the laws of derivatives, something with zero time derivative is constant.

So there is a logical equivalence between zero net force on a body and that body having constant velocity.

But, velocity's value has nothing to do with that definition. So while an object at constant zero velocity does not have a net acceleration, the fact that it has no net acceleration does not imply that its constant velocity is, in fact, zero.

Therefore E is wrong.
NoCleverName
QUOTE (rpenner+Nov 24 2010, 08:42 PM)
So while an object at constant zero velocity does not have a net acceleration, the fact that it has no net acceleration does not imply that its constant velocity is, in fact, zero.

Save for the case, as muddlerunner might point out, of a rock sitting still on earth's surface under the acceleration of gravity.
mudderrunner
QUOTE (NoCleverName+Nov 25 2010, 09:27 AM)
Save for the case, as muddlerunner might point out, of a rock sitting still on earth's surface under the acceleration of gravity.

nope. The rock is accelerating due to the ground not letting it resume uniform motion.

nice try notclever
boit
QUOTE (mudderrunner+Nov 25 2010, 07:28 PM)
nope. The rock is accelerating due to the ground not letting it resume uniform motion.

nice try notclever

nope. The rock is on an accelerated frame but has no net acceleration (i.e. zero net acceleration) due to the opposing effect of the surface.
Beer w/Straw
A decrease in the net force doesn't mean that the resulting net force is balanced differently than before.

The question or possible answers are meaningless.

:EDIT:

Ooops it said the net for 'ceases', not 'decreases.'

mtoom
What I wrote above was wrong. I think I understand first order logic, I'm not a *****, but thanks for the notes.

| ------------------------------------------------------
| QUESTION:
|
| If the net force on an object ceases, then the object __________.
|
| A. decelerates
| B. coasts at constant speed
| C. accelerates
| D. doesn’t obey Newton’s laws anymore
| E. has zero speed
| ------------------------------------------------------

Observation 1. Option A, C and D are clearly wrong.
Observation 2. After the net force on an object ceases, the object may have no speed or have some speed (not enough information to tell which).
Observation 3. Option B includes the word "coast" which implies after the net force ceases, the speed of the object is not zero.
Observation 4. Option E implies that after the net force ceases, the speed of the object is not zero.

Conclusion: If you accept the premises of observations 2, 3 and 4, then it logically follows that both options B and E are too narrow to be correct (the logical OR of Option B and Option E would be correct).
rpenner
I would say B is more correct than E, and would consider that either "coasting" at zero speed is allowed as an extension to any conventional definition or the special case of an object that often coasts, like a car or an air hockey puck, is being considered from it's conceptual start with a net force in only one direction. "Coast", like "move" is less formal language than "constant speed" and it's only purpose in the sentence is to attach the language "constant speed" to the rest of the question.

In formal language, the question is

Given a Newtonian-modeled, point particle of mass m, whose time evolution of position may be spoken of as x(t), for all times after time t₀ the sum of all forces that act on the particle are zero. What may be said about the time evolution of position for times after time t₀ ?
A. ⊦ t > t₀ ⇒ (∂²x(t)/ ∂t²) · (∂x(t)/ ∂t) < 0.
B. ⊦ t > t₀ ⇒ | ∂²x(t)/ ∂t² | = 0.
C. ⊦ t > t₀ ⇒ (∂²x(t)/ ∂t²) · (∂x(t)/ ∂t) > 0.
D. ⊦ t > t₀ ⇒ m (∂²x(t)/ ∂t²) ≠ 0.
E. ⊦ t > t₀ ⇒ | ∂x(t)/ ∂t | = 0.

So your hang up on the term "coasts" seems to be without scholarly merit. It's just an informal way to say "has a time evolution of position" or "has a motion" and physics allows something have a motion described by zero speed, which is a special case of constant speed.
mudderrunner
QUOTE (boit+Nov 25 2010, 03:05 PM)
nope. The rock is on an accelerated frame but has no net acceleration (i.e. zero net acceleration) due to the opposing effect of the surface.

there is no opposing force. All the force is in the direction of up being applied by ground. Gravity is not a force. It's just uniform motion in curved space. So net acceleration is not zero.

B. and E. can both be right depending on the frame of reference. The other answers are definitely wrong. And since the question wasn't asked from a relative frame of reference then there can be no 'one' correct answer.
mtoom
QUOTE
"Coast", like "move" is less formal language than "constant speed" and it's only purpose in the sentence is to attach the language "constant speed" to the rest of the question.

I disagree. Every definition of the word "coast" in Websters implies a moving object. If they said "has constant speed" then that would be right.

QUOTE (->
 QUOTE "Coast", like "move" is less formal language than "constant speed" and it's only purpose in the sentence is to attach the language "constant speed" to the rest of the question.

I disagree. Every definition of the word "coast" in Websters implies a moving object. If they said "has constant speed" then that would be right.

–verb (used without object)
7.
to slide on a sled down a snowy or icy hillside or incline.
8.
to descend a hill or the like, as on a bicycle, without using pedals.
9.
to continue to move or advance after effort has ceased; keep going on acquired momentum: We cut off the car engine and coasted for a while.
10.
to advance or proceed with little or no effort, esp. owing to one's actual or former assets, as wealth, position, or name, or those of another: The actor coasted to stardom on his father's name.
11.
to sail along, or call at the various ports of, a coast.
12.
Obsolete . to proceed in a roundabout way.
–verb (used with object)
13.
to cause to move along under acquired momentum: to coast a rocket around the sun.
14.
to proceed along or near the coast of.
15.
Obsolete . to keep alongside of (a person moving).
16.
Obsolete . to go by the side or border of.

If the phrasing of B implies that the object is moving, then B is too narrow of a definition and is therefore wrong.

They should have used the word "has", so B would read out as: b. has constant velocity. Then, B is fully correct as E is necessary but not sufficient and B includes E.
mtoom
QUOTE
I would say B is more correct than E

I agree. But, by saying this you concede that neither are fully correct, which is my only complaint regarding this question.

QUOTE (->
 QUOTE I would say B is more correct than E

I agree. But, by saying this you concede that neither are fully correct, which is my only complaint regarding this question.

So your hang up on the term "coasts" seems to be without scholarly merit. It's just an informal way to say "has a time evolution of position" or "has a motion"

My hang up on the term "coasts" is not question of physics as much as it is of English. Idiomatically, in English, an object which is coasting is NOT static. For the purposes of the question, this feature (implies that the object has non-zero speed, and) renders the option B invalid as a complete answer.

Can we agree on that much?
rpenner
No, I cannot agree. It's an example of artificial rigor and unhelpful pedantry.

Despite having entries which are commonly refered to as definitions, dictionaries do not define words -- they seek to capture the ways in which words are used. Here the author seeks a short phrase to capture the details of a physics model. Pedagogically the intent of the exam writer is clear.

"If you're the kind of person who insists on this or that 'correct' use... abandon your pedantry as I did mine. Dive into the open flowing waters and leave the stagnant canals be... Above all, let there be pleasure!"— Attributed to Stephen Fry
NoCleverName
QUOTE (mudderrunner+Nov 25 2010, 12:28 PM)
nope. The rock is accelerating due to the ground not letting it resume uniform motion.

nice try notclever

Don't worry, I misread a "nuance" I thought I saw in the problem statement that allowed "E" as a "trick" answer.
Beer w/Straw
QUOTE (mtoom+Nov 26 2010, 03:31 AM)
I disagree. Every definition of the word "coast" in Websters implies a moving object. If they said "has constant speed" then that would be right.

If I define speed as distance/time there is a restriction on time being zero not distance being zero. B includes E iff the initial speed is zero.
boit
QUOTE (Beer w/Straw+Nov 26 2010, 07:04 PM)
If I define speed as distance/time there is a restriction on time being zero not distance being zero. B includes E iff the initial speed is zero.

I wanted to say that speed can only be expressed as distance/time then I remembered photons. They shoot at c once they come into existance. You know, light can not travel at a speed other than c. Maybe it creates its own time as it forms? If time was created after the big bang, I can clearly understand how matter suddenly was flung far apart in apparently no time. Matter creating pacetime as it moves along just as a bullet drills its own hole throug stuff.
mudderrunner
QUOTE (NoCleverName+Nov 26 2010, 07:01 AM)
Don't worry, I misread a "nuance" I thought I saw in the problem statement that allowed "E" as a "trick" answer.

the problem does allow E as an answer, as well as B. This is because it is not asked from a relative frame of reference. So from a point of view it could be moving uniformly or not moving, but not accelerating.
rpenner
That's precisely the type of overthinking that sunk the OP.
Capracus
QUOTE (Beer w/Straw+Nov 26 2010, 04:04 PM)
If I define speed as distance/time there is a restriction on time being zero not distance being zero. B includes E iff the initial speed is zero.
As moot pointed out, coast implies initial motion, so under B the object could not be at rest, and therefore could not contain E.

If condition D is rejected, and Newton's first law of motion remains valid, then both B and E could be correct.
Beer w/Straw
QUOTE (Capracus+Nov 27 2010, 05:18 AM)
As moot pointed out, coast implies initial motion,

So does speed, doesn't it?
rpenner
Speed is a number in the domain of physics, the absolute value of the velocity vector.

"coasts at constant speed" is just the exam author's informal way of saying "from that time on has unchanging speed." It's a little better than "moves at constant speed" because coasts specifically refers to states of motion when effort is not being expended. In physics, not moving and moving are both valid states of motion, expressed by different values of the speed. Being deliberately obtuse about the test maker's pedagogical intent suggests you aren't doing your share to achieve a meeting of the minds.
mudderrunner
QUOTE (rpenner+Nov 27 2010, 12:13 AM)
That's precisely the type of overthinking that sunk the OP.

I think it's contradictory to ask a student to not overthink something, especially when it doesn't produce an incorrect answer. While the teacher may have been looking for B, it would be equally correct to consider E. In fact, the lack of emphasis on relativity by science teachers trying to make things "easier" to understand is often what creates confusion.

It's like telling students that the bowling ball and feather will land at the same time, negating wind resistance. It's likely wrong in non-perfect conditions and absolutely wrong in perfect conditions. Suddenly they are thinking about gravity geocentrically and may have trouble in more advanced classes.
Capracus
QUOTE (rpenner+Nov 27 2010, 05:43 PM)
In physics, not moving and moving are both valid states of motion, expressed by different values of the speed. Being deliberately obtuse about the test maker's pedagogical intent suggests you aren't doing your share to achieve a meeting of the minds.
Label me dunce if it pleasures you, but I've always understood motion as a change in position, and a lack of such state to be motionless or at rest. The original question appears to offer four distinct options relating to motion:

A. Reduction in speed
B. Coast (positive speed) at a constant speed.
C. Increase in speed.
E. No speed, or rest.

Why would it not be the test maker's intent to define coast as non zero speed in an effort to more fully illustrate the conditions of motion?
Beer w/Straw
First law: Every body remains in a state of rest or uniform motion (constant velocity) unless it is acted upon by an external unbalanced force. [2][3][4] This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant speed in a straight line.

http://en.wikipedia.org/wiki/First_law_of_motion

If answer B does not include E for 0 speed, then none of the answers is ultimately the correct answer. Unless you disregard Newton altogether.

QUOTE
I analyze the packing of cornflakes according to strict general relativistic considerations modified by first and second order quantum corrections in hbar- in a fully iterative self-consistent way, being careful to account for non-Euclidean metrics over the volume of the cornflake box.

rpenner
Oh yes, it's a suboptimal wording. But in an exam handed to you to complete, you pick the best legal answer and let it pass. They aren't trying to "trick" you -- they are trying to measure your ability to apply basic Newtonian principles. And by overparsing the question, you set yourself up for failure.

Save the overthinking for addenda to your essay questions if there is still time in the exam period.
mudderrunner
QUOTE (rpenner+Nov 27 2010, 08:22 PM)
Oh yes, it's a suboptimal wording. But in an exam handed to you to complete, you pick the best legal answer and let it pass. They aren't trying to "trick" you -- they are trying to measure your ability to apply basic Newtonian principles. And by overparsing the question, you set yourself up for failure.

Save the overthinking for addenda to your essay questions if there is still time in the exam period.

why are you more inclined to fault the student rather then the teacher? Teachers are certainly not infallible. I don't think it is over thinking at all. There are two correct answers based on newtons laws. Even galileo would have said the same.
rpenner
The "correct" answer as defined by pedants who want a different connection verb than "coasts" in "coasts at constant speed" is that the speed (indeed the velocity!) does not change after the net force falls to zero. That represents a quibble with "coast", with "constant" (since it can't be constant just part of the time, right?) and "speed" (a concept too primitive for even 1-D physics). This is not offered as a choice, because the exam is written for physicists and not pedants.

"B or E" is also not offered as a choice.

"B and E" is just wrong -- criminal in fact.

B is more correct than E, because E is not correct in infinitely more ways than B is not correct -- if you pedantically insist, post hoc that an object can't coast at zero speed. However, there are many, many examples of times when physicists utterly neglect cases of measure zero.

Experimentally, there is no way to distinguish between coasting at a very, very low speed and not moving at all.

Further the OP stated that the instructor was not versed in physics, and this smells like an attempt to bully a teacher into grade inflation.

Part of becoming an adult is learning to live in a world that doesn't cater to you, and many times there will be no "perfect" answer to life's problems. That doesn't mean you get to opt out of trying to answer them the best that you can.

And so B is the physicists choice.
mudderrunner
QUOTE (rpenner+Nov 27 2010, 08:58 PM)
The "correct" answer as defined by pedants who want a different connection verb than "coasts" in "coasts at constant speed" is that the speed (indeed the velocity!) does not change after the net force falls to zero. That represents a quibble with "coast", with "constant" (since it can't be constant just part of the time, right?) and "speed" (a concept too primitive for even 1-D physics). This is not offered as a choice, because the exam is written for physicists and not pedants.

"B or E" is also not offered as a choice.

"B and E" is just wrong -- criminal in fact.

B is more correct than E, because E is not correct in infinitely more ways than B is not correct -- if you pedantically insist, post hoc that an object can't coast at zero speed. However, there are many, many examples of times when physicists utterly neglect cases of measure zero.

Experimentally, there is no way to distinguish between coasting at a very, very low speed and not moving at all.

Further the OP stated that the instructor was not versed in physics, and this smells like an attempt to bully a teacher into grade inflation.

Part of becoming an adult is learning to live in a world that doesn't cater to you, and many times there will be no "perfect" answer to life's problems. That doesn't mean you get to opt out of trying to answer them the best that you can.

And so B is the physicists choice.

if "b or e" then it could also be "b and e" since both imply that the frame of reference is not established.

b is not more correct then e. The question does not make reference to quantities of situations. B is a state of motion. E is a state of motion. They are equally correct in this regard.

It seemed more like the OP was talking to a 3rd party instructor rather then the test giver. Why would a non-physics professor be teaching physics? emphasis on "professor".

This is not some sort of life lesson question. And if it was it would be that adults make mistakes and you shouldn't be afraid to call them on it.

Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 01:14 AM)
If answer B does not include E for 0 speed, then none of the answers is ultimately the correct answer. Unless you disregard Newton altogether.
If option B omitted coasts, and instead stated continue at constant speed, I would totally agree. To me coasts implies a speed greater than zero, which would exempt option E.
Capracus
QUOTE (rpenner+Nov 28 2010, 01:58 AM)
Experimentally, there is no way to distinguish between coasting at a very, very low speed and not moving at all.
Under Newtonian laws, or any other that I'm aware of, a condition of zero speed is not possible in reality, and would only exist abstractly in an exercise such as this exam question. So in this regard B would be the only correct answer.

mudderrunner
QUOTE (Capracus+Nov 27 2010, 10:01 PM)
Under Newtonian laws, or any other that I'm aware of, a condition of zero speed is not possible in reality, and would only exist abstractly in an exercise such as this exam question. So in this regard B would be the only correct answer.

if the answer applies to the exam, why would you negate it as an answer to the exam?
Capracus
QUOTE (mudderrunner+Nov 28 2010, 03:06 AM)
if the answer applies to the exam, why would you negate it as an answer to the exam?
The exam being an abstract exercise allows for unrealistic conditions. The object in question would have to exist alone in its own universe to be without any net force acting on it, or in a universe where all matter is physically locked in a state of rest. If the exam question was meant to reflect known reality, then I would choose answers A or C, because neither rest or constant speed would be possible. If it was intended as an abstract examination of Newton's first law of motion, I would pick answers B and E.
Beer w/Straw
QUOTE (mtoom+Nov 24 2010, 10:18 PM)
D. doesn’t obey Newton’s laws anymore

I shouldn't post this but, doesn't answer d assume that it obeyed Newton's laws in the first place?
mudderrunner
QUOTE (Capracus+Nov 27 2010, 10:57 PM)
The exam being an abstract exercise allows for unrealistic conditions. The object in question would have to exist alone in its own universe to be without any net force acting on it, or in a universe where all matter is physically locked in a state of rest. If the exam question was meant to reflect known reality, then I would choose answers A or C, because neither rest or constant speed would be possible. If it was intended as an abstract examination of Newton's first law of motion, I would pick answers B and E.

so again, why would E be incorrect if it applies and is correct?
Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 04:07 AM)
I shouldn't post this but, doesn't answer d assume that it obeyed Newton's laws in the first place?
Newton's laws as applied to what condition, reality or an abstract question? As I noted earlier, zero speed and constant speed are abstractions, with no known basis in reality.

QUOTE (mudderrunner+)
so again, why would E be incorrect if it applies and is correct?
If under the conditions of the question, zero speed were possible, then E could be correct.
Beer w/Straw
QUOTE (Capracus+Nov 28 2010, 05:24 AM)
Newton's laws as applied to what condition, reality or an abstract question? As I noted earlier, zero speed and constant speed are abstractions, with no known basis in reality.

I just gave argument that all available answers are incorrect if you want to get ultra picky.
Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 05:33 AM)
I just gave argument that all available answers are incorrect if you want to get ultra picky.
I'm not trying to be picky. I realize as rpenner pointed out that there is an academically appropriate answer, but that's no reason not to explore the intended, or possible conditions of the question. What do think are the implied conditions of the question?
Beer w/Straw
QUOTE (Capracus+Nov 28 2010, 05:53 AM)
What do think are the implied conditions of the question?

Given this thread has gone this far, I'm starting to think the question was a joke.
Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 05:58 AM)
Given this thread has gone this far, I'm starting to think the question was a joke.

Here is an example of a similar exam question.
QUOTE
13. The sum of the forces acting on an object is zero. Which statements could describe the object’s motion?

I. At rest
II. Moving in a straight line at constant speed
III. Moving in a circle at constant speed.

A. only I
B. only I and II
C. only I and III
D. only II and III
E. all of the above
http://www.shs.d211.org/physicsd211/quizze...ces/Unit1MC.htm
The correct answer above is B, which corresponds to answers B and E in our question.
rpenner
But it is a different question, since the word "which could" is used, and not "if ... then ...".
boit
QUOTE
The only thing that it being daytime proves is that the sun is above the horizon.
I totally agree. Not so the author or authors of the book of Genesis. Apparently they had day and night times well before the sun was even created. Unless of course the sun was hidden from view in which case it was revealed and not created, just like the water the spirit was hovering over.
Beer w/Straw
QUOTE (Capracus+Nov 28 2010, 06:59 AM)
Here is an example of a similar exam question....

The correct answer above is B, which corresponds to answers B and E in our question.

Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 03:19 PM)
That the original exam question was legitimate. That both questions are examples of hypotheses that imply the given conclusions.
Beer w/Straw
QUOTE (Capracus+Nov 28 2010, 11:14 PM)
That the original exam question was legitimate.

How do I know that the original poster didn't take an existing question and worded it differently?

If it was a test question, and you were only allowed to choose one for your answer, what would you choose?
Capracus
QUOTE (Beer w/Straw+Nov 28 2010, 11:56 PM)
How do I know that the original poster didn't take an existing question and worded it differently?

If it was a test question, and you were only allowed to choose one for your answer, what would you choose?
Since a definitional stretch of option B could be used to include option E, I would choose B.
Beer w/Straw
QUOTE (Capracus+Nov 29 2010, 12:12 AM)
Since a definitional stretch of option B could be used to include option E, I would choose B.

And if it didn't, would you still choose B?
mudderrunner
QUOTE (Capracus+Nov 28 2010, 07:12 PM)
Since a definitional stretch of option B could be used to include option E, I would choose B.

the definition stretch does not exist. I assume it's a subtle personal bias that makes it seem like it does. The two answers are equally correct. They differ only by frame of reference and the question does not have one.
Beer w/Straw
QUOTE (mudderrunner+Nov 29 2010, 12:21 AM)
the definition stretch does not exist. I assume it's a subtle personal bias that makes it seem like it does. The two answers are equally correct. They differ only by frame of reference and the question does not have one.

mudderrunner
QUOTE (Beer w/Straw+Nov 28 2010, 07:28 PM)

either randomly fill in b. or e. and forget about it cause the test isn't very important to my grade, or see the professor after class.
Beer w/Straw
You don't sound too confident if you have to speak to the professor now.

mudderrunner
QUOTE (Beer w/Straw+Nov 28 2010, 07:37 PM)
You don't sound too confident if you have to speak to the professor now.

You misunderstood. I would speak to him about how dumb his question was and make sure he doesn't mark it wrong because of his mistake.
mtoom
QUOTE
Given this thread has gone this far, I'm starting to think the question was a joke.

QUOTE (->
 QUOTE Given this thread has gone this far, I'm starting to think the question was a joke.

How do I know that the original poster didn't take an existing question and worded it differently?

I can assure you the question is real. It is from an earth science class on natural disasters at the University of British Columbia in Vancouver, Canada.
mtoom
QUOTE (rpenner+Nov 26 2010, 04:41 AM)
No, I cannot agree. It's an example of artificial rigor and unhelpful pedantry.

Despite having entries which are commonly refered to as definitions, dictionaries do not define words -- they seek to capture the ways in which words are used. Here the author seeks a short phrase to capture the details of a physics model. Pedagogically the intent of the exam writer is clear.

"If you're the kind of person who insists on this or that 'correct' use... abandon your pedantry as I did mine. Dive into the open flowing waters and leave the stagnant canals be... Above all, let there be pleasure!"— Attributed to Stephen Fry

I understand your point here and agree in principle.

But I disagree that the notion of the author is "pedagogically clear".

The question creates confusion due to the wording. In this instance, I am the student. For me, the problem with B versus E was immediately apparent, causing confusion.

Thus, it is not "pedagogically clear".

This was all due to the wording of the question. It probably was worded that way to make it "easier for the student" but really just resulted in the situation we have here. Yes, if I interpret the question "loosely" and go with the "best available answer" I would probably pick B.

mudderrunner perfectly hits on the critical issue for me, regarding this question, with the following remarks:

QUOTE
I think it's contradictory to ask a student to not overthink something, especially when it doesn't produce an incorrect answer. While the teacher may have been looking for B, it would be equally correct to consider E. In fact, the lack of emphasis on relativity by science teachers trying to make things "easier" to understand is often what creates confusion.
mtoom
QUOTE (Capracus+Nov 27 2010, 05:18 AM)
As moot pointed out, coast implies initial motion, so under B the object could not be at rest, and therefore could not contain E.

If condition D is rejected, and Newton's first law of motion remains valid, then both B and E could be correct.

QUOTE
As moot pointed out, coast implies initial motion, so under B the object could not be at rest, and therefore could not contain E.

No! Wrong!

"coast" implies motion, but not initial motion.

Answers A,B,C,D,E refer to what happens to the object when the net force ceases. Not before the net force ceases.

Further even if it did imply initial motion, the object may not be moving at the point that the net force = 0. Review my initial example (in my second post on the thread).
mtoom
QUOTE (Beer w/Straw+Nov 28 2010, 04:07 AM)
I shouldn't post this but, doesn't answer d assume that it obeyed Newton's laws in the first place?

lol
Beer w/Straw
QUOTE (mudderrunner+Nov 29 2010, 12:50 AM)
You misunderstood. I would speak to him about how dumb his question was and make sure he doesn't mark it wrong because of his mistake.

I was thinking about someone else at the time when I asked actually.
Beer w/Straw
And 'overthinking' something should be more reserved for assignments rather than a test IMO.
Capracus
QUOTE (mtoom+Nov 29 2010, 02:06 AM)

No! Wrong!

"coast" implies motion, but not initial motion.

Answers A,B,C,D,E refer to what happens to the object when the net force ceases. Not before the net force ceases.

Further even if it did imply initial motion, the object may not be moving at the point that the net force = 0. Review my initial example (in my second post on the thread).
How would an object continue non zero velocity unless it were in that initial state of motion?

QUOTE (mtoom+)

QUESTION:
|
| If the net force on an object ceases, then the object __________.
|
| A. decelerates
| B. coasts at constant speed
| C. accelerates
| D. doesn’t obey Newton’s laws anymore
| E. has zero speed| ------------------------------------------------------

Observation 1. Option A, C and D are clearly wrong.
Observation 2. After the net force on an object ceases, the object may have no speed or have some speed (not enough information to tell which).
Observation 3. Option B includes the word "coast" which implies after the net force ceases, the speed of the object is not zero.
Observation 4. Option E implies that after the net force ceases, the speed of the object is not zero.
Observation 4. Option E implies that after the net force ceases, the speed of the object is not zero?

mtoom
Capracus,

I misinterpreted what you wrote. I dont think we disagree on your remarks
Capracus
QUOTE (mudderrunner+Nov 29 2010, 12:21 AM)
the definition stretch does not exist. I assume it's a subtle personal bias that makes it seem like it does. The two answers are equally correct. They differ only by frame of reference and the question does not have one.
In answer B, if the meaning of coast is narrowly interpreted as continuation without effort, then it could apply to a condition of zero velocity and include E.

Answer E being more rigidly defined could not be interpreted to include continuation of non zero velocity.
Capracus
QUOTE (mtoom+Nov 29 2010, 01:43 AM)
I can assure you the question is real. It is from an earth science class on natural disasters at the University of British Columbia in Vancouver, Canada.
Did the conditions of the exam allow for multiple answers for this question?
mudderrunner
QUOTE (Capracus+Nov 28 2010, 10:46 PM)
In answer B, if the meaning of coast is narrowly interpreted as continuation without effort, then it could apply to a condition of zero velocity and include E.

Answer E being more rigidly defined could not be interpreted to include continuation of non zero velocity.

Forget about "continuation". Coasts means to move at constant velocity. And besides, the question implies change, not continuation. Net force ceased. This is a change. There was a net force, and then suddenly there wasn't. It is impossible for the same motion to occur after the net force ceases as compared to before.

B. a relative motion
E. no relative motion

They are both rigidly defined.
rpenner
Such rigidity will cause you to snap like a twig.
Capracus
QUOTE (mudderrunner+Nov 29 2010, 04:07 AM)
Forget about "continuation". Coasts means to move at constant velocity.
Actually it means to proceed without additional input, and constant velocity would depend on the absence of opposing force.

QUOTE
And besides, the question implies change, not continuation. Net force ceased. This is a change. There was a net force, and then suddenly there wasn't.
The exam question is a restatement of Newton's first law of motion, which implies that absent net force, objects in motion remain in motion, and objects at rest remain at rest. In other words, if an object was in motion before the cessation of force as the question states, then it continues to be afterwords. Objects not in motion before, remain so after.

QUOTE (->
 QUOTE And besides, the question implies change, not continuation. Net force ceased. This is a change. There was a net force, and then suddenly there wasn't.
The exam question is a restatement of Newton's first law of motion, which implies that absent net force, objects in motion remain in motion, and objects at rest remain at rest. In other words, if an object was in motion before the cessation of force as the question states, then it continues to be afterwords. Objects not in motion before, remain so after.

It is impossible for the same motion to occur after the net force ceases as compared to before.
It's not a question of like motion, but of motion or no motion. Angular motion becomes straight, and no motion stays put.

QUOTE

B. a relative motion
E. no relative motion

They are both rigidly defined.
Personally I suspect that the author of the question intended B and E to reflect the above conditions, but if one had to choose the most correct, I think that B allows for some weasel room.
Beer w/Straw
mudderrunner
QUOTE (Capracus+Nov 29 2010, 01:32 AM)
Actually it means to proceed without additional input, and constant velocity would depend on the absence of opposing force.

That sounds congruent with what I said. The only addition I would make is regarding "proceed". More accurately it is "proceed from that point on" considering that the coasting happens at the point when net force ceases. At that point I don't care what you call it, "without additional input" or "absence of net force" are the same. When in context it is obviously more appropriate to say that latter.

QUOTE
The exam question is a restatement of Newton's first law of motion, which implies that absent net force, objects in motion remain in motion, and objects at rest remain at rest. In other words, if an object was in motion before the cessation of force as the question states, then it continues to be afterwords. Objects not in motion before, remain so after.

actually no. The second part of this statement is false. The state of motion before and after the net force can be different. It could have zero motion, then a net force occurs, then it ceases, then the object will have constant velocity. Or... it could start with constant velocity, then a net force occurs, then it ceases, then the object could have zero motion.

QUOTE (->
 QUOTE The exam question is a restatement of Newton's first law of motion, which implies that absent net force, objects in motion remain in motion, and objects at rest remain at rest. In other words, if an object was in motion before the cessation of force as the question states, then it continues to be afterwords. Objects not in motion before, remain so after.

actually no. The second part of this statement is false. The state of motion before and after the net force can be different. It could have zero motion, then a net force occurs, then it ceases, then the object will have constant velocity. Or... it could start with constant velocity, then a net force occurs, then it ceases, then the object could have zero motion.

It's not a question of like motion, but of motion or no motion. Angular motion becomes straight, and no motion stays put.

it IS a question of like motion when you use the term "continue" or "proceed" like you are doing. Don't lose sight of what my response was actually responding to.

QUOTE

Personally I suspect that the author of the question intended B and E to reflect the above conditions, but if one had to choose the most correct, I think that B allows for some weasel room.

I think you are making this statement with respect to how the professor would grade you. Because otherwise, they are both equally correct. The professor is an idiot.
NoCleverName
This is starting to sound like the worst of the "Plane on a Conveyor" Thread ... why not just quit now?
PhysOrg scientific forums are totally dedicated to science, physics, and technology. Besides topical forums such as nanotechnology, quantum physics, silicon and III-V technology, applied physics, materials, space and others, you can also join our news and publications discussions. We also provide an off-topic forum category. If you need specific help on a scientific problem or have a question related to physics or technology, visit the PhysOrg Forums. Here you’ll find experts from various fields online every day.