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Your fellow human (yfh)
Why does gravity attract?

Why did so many planets rotate around stars instead of being flung off out of any rotation?

If star and planet both have gravity, why would one attract to the other? Magnets only attract to opposites, but is gravity always possitive, negative or the same?
blue_bottle
Gravity comes from the bending of space time. In a very simple sense, imagine a black bin liner which is attached at the corners to some surface to hold it up in the air. Then place a ball in the middle.

user posted image

This causes it to bend around the object, as in the diagram. Know imagine placing a pea on the edge at some point. The pea would role down towards the ball. this is gravity, pulling Earth (the pea) towards the sun (the ball), as shown in the diagram. But the Earth is moving VERY FAST therefore it keeps going round the sun with the forces around it balanced.

Hope this answers your question. I'm a bit unsure as to my explanation. tongue.gif
El_Machinae
We only think that a lot of planets orbit stars - but I'd imagine that in truth there are many, many more planets on route between stars (flung by a supernova). They can loosely be described as being in orbit, because they are affected by the stars they are travelling by, but if we were to look at them we wouldn't think of them as being in orbit.
Issachar
QUOTE (Your fellow human (yfh)+Mar 15 2006, 08:22 AM)
Why did so many planets rotate around stars instead of being flung off out of any rotation?


That is a good question YFH. smile.gif I have been fascinated myself wondering about this from time to time.
Thanks for the illustration blue_bottle, that helps a lot. rolleyes.gif

There is the centripetal or center-seeking force which is the gravitational force for a planet in orbit. This is balanced by the the or center-fleeing force which displaces it radially from the center of the path.

So how does the centrifugal force act on the source to create the balance?
I can see how the pea would spin around the ball on the liner as would a basketball spin around the rim before falling through the basket. But what causes the planet to spin?
How does the expanding universe affect what is going on in this scenario? huh.gif
lostinjersey
The expanding universe doesn't affect the rotation of planets around stars.

Gravity does not have a negative and a positive it only attracts. Anti-gravity is the opposite, it only repels.

planets stay moving around stars because gravity holds it there. think about it this way: say you have a yoyo and you swing it around in a circle really fast. the string acts like gravity in that it keeps the yoyo from flying off it's path. It pulls the yoyo in toward the center while the speed of the yoyo pushes it foward creating a circular path.
Blue bottle's pic is actually very helpful. The bending of space time is another nice way of picturing what gravity is doing.
Issachar
QUOTE (lostinjersey+Mar 22 2006, 06:51 PM)
The expanding universe doesn't affect the rotation of planets around stars.


I should have asked more appropriately 'Does the expanding universe affect what is going on in this scenario?' instead of 'How Does'.
Thanks for answering my question lostinjersey, you provided an excellent example of the string of the yo-yo acting like the gravity on the planets which I have considered before.

Thats is fine that the expanding universe doesn't affect the rotation of planets around stars, but why is this the case? Is it because the increased distance from expansion is offset precisely by a slight decrease in the gravitational force or am I missing something? Just curious so thanks in advance!
There is an extreme amount of balance going on amidst all the chaos. unsure.gif
atfpcop
Lost in jersey,

I get the yo yo idea, but i still am having a hard grasp of this, Gravity is related to mass right, If this is the case what really creates the force, the way i see it and i am probably off here but spin a ball and what ever is not attached to the ball will fly off the surface, Our gravitational pull is strong enough to take big rockets to get free of, yet we stand on the planet and don't go any where.
blue_bottle

As I showed with the diagram above, gravity is caused by an objects mass bending space time. The greater the mass, the greater the bend in space time. The spinning ball analogy does not create a centripetal (can someone please tell me if this is the right one! blink.gif ) to hold it down to the balls surface. Gravity is the glue you'd have to use to keep an object attached to a spinning ball.

Hope this helps you understand it a bit better. See my above picture for a better visual recognition (bear in mind this is a 3D representation of what is essentially a 4D thing)
atfpcop
Blue bottle i get the solar system part but i dont get how earth spinning can cause our gravity to keep us on that is what i was asking, sorry should have been more specific in my question.
J-n
QUOTE (Issachar+Mar 22 2006, 05:48 PM)
QUOTE (lostinjersey+Mar 22 2006, 06:51 PM)
The expanding universe doesn't affect the rotation of planets around stars.


I should have asked more appropriately 'Does the expanding universe affect what is going on in this scenario?' instead of 'How Does'.
Thanks for answering my question lostinjersey, you provided an excellent example of the string of the yo-yo acting like the gravity on the planets which I have considered before.

Thats is fine that the expanding universe doesn't affect the rotation of planets around stars, but why is this the case? Is it because the increased distance from expansion is offset precisely by a slight decrease in the gravitational force or am I missing something? Just curious so thanks in advance!
There is an extreme amount of balance going on amidst all the chaos. unsure.gif

If I'm not mistaken the reason would be that it effects all things equally, not only is the space between the earth and the sun has increased, but the size of the objects as well. The expansion occurs within each atom as well as on the macro scale..

I hope this is right/helpful smile.gif ... but probally not.. LoL

J-n
negative1
Ok i'm having trouble with the illustration by blue bottle.

If this was true would the ball in the middle represent the sun and the pea the earth?
That only show for our solar system but this illustration is supposed to be the entire curvature of the universe I dont see how if we're located say on one side of the ridge how the pull would causes us to orbit the sun rather than just continue to speed toward the "lowest" point.

Also how does this example explain gravity in general say the minute amount of gravity between all objects that exsist.

I'm not looking for a where does gravity come from there are plenty of threads on that just an explaination of this explaination.

Anyone able to explain this?
00DB
QUOTE (blue_bottle+Mar 22 2006, 08:59 AM)
Gravity comes from the bending of space time. In a very simple sense, imagine a black bin liner which is attached at the corners to some surface to hold it up in the air. Then place a ball in the middle.

user posted image

This causes it to bend around the object, as in the diagram. Know imagine placing a pea on the edge at some point. The pea would role down towards the ball. this is gravity, pulling Earth (the pea) towards the sun (the ball), as shown in the diagram. But the Earth is moving VERY FAST therefore it keeps going round the sun with the forces around it balanced.

Hope this answers your question. I'm a bit unsure as to my explanation. tongue.gif


I've always wondered about this.... this makes sence, in a 2D prespective, but what about a 3 dimentional prespective...? this works on a surface, but there is no surface, its space.... so it would pull the fabric of space in ALL directions, not just one dimetional (like the photo), wich would cause an object to not bend around another object... cause space would be bent all around it, not just from the side, but from everywhere...

Max420
I am not even sure what the question is anymore, at first it was pretty clear, but now i'm lost blink.gif
Guest
QUOTE (00DB+Apr 19 2006, 05:47 AM)
QUOTE (blue_bottle+Mar 22 2006, 08:59 AM)
Gravity comes from the bending of space time. In a very simple sense, imagine a black bin liner which is attached at the corners to some surface to hold it up in the air. Then place a ball in the middle.

user posted image

This causes it to bend around the object, as in the diagram. Know imagine placing a pea on the edge at some point. The pea would role down towards the ball. this is gravity, pulling Earth (the pea) towards the sun (the ball), as shown in the diagram. But the Earth is moving VERY FAST therefore it keeps going round the sun with the forces around it balanced.

Hope this answers your question. I'm a bit unsure as to my explanation. tongue.gif


I've always wondered about this.... this makes sence, in a 2D prespective, but what about a 3 dimentional prespective...? this works on a surface, but there is no surface, its space.... so it would pull the fabric of space in ALL directions, not just one dimetional (like the photo), wich would cause an object to not bend around another object... cause space would be bent all around it, not just from the side, but from everywhere...

I've always wondered that, in all the examples i've seen its always two dimensional. How does gravity affect spacetime in a three dimensional sense?
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