A question about gravity

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I'm not very knowledgeable on this subject and it's well beyond my understanding, but from what I do understand objects do not fall, it is the earth crashing into them because the earth is rotating and travelling through space, but what happens if you stand directly on the north or south pole and drop an apple, you are out of earths path and spin, so isn't the apple actually falling?
 

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  • #2
russ_watters
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Welcome to PF!
I'm not very knowledgeable on this subject and it's well beyond my understanding, but from what I do understand objects do not fall, it is the earth crashing into them because the earth is rotating and travelling through space...
Sorry, no, that isn't correct. In the Newtonian explanation, objects fall because gravity pulls objects with mass toward each other. In Relativity, space is warped by the presence of mass, pulling/pushing objects together. None of that has anything to do with motion/rotation.
 
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Welcome to PF!

Sorry, no, that isn't correct. In the Newtonian explanation, objects fall because gravity pulls objects with mass toward each other. In Relativity, space is warped by the presence of mass, pulling/pushing objects together. None of that has anything to do with motion/rotation.
Brian Greene discusses it briefly in this video from about 9.20.

 
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Earth's rotation has nothing to do with gravity, why do you think it should?
I can understand your concept about the Earth moving through space causing it collide with other stuff, although that's also wrong
If that were true then only stuff ahead of the leading edge would fall on to Earth, while on the trailing edge stuff that wasn't tied down would float away.
 
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Earth's rotation has nothing to do with gravity, why do you think it should?
I can understand your concept about the Earth moving through space causing it collide with other stuff, although that's also wrong
If that were true then only stuff ahead of the leading edge would fall on to Earth, while on the trailing edge stuff that wasn't tied down would float away.

That's just how I've come to understand how gravity works, but I thougt it was wrong.

In the video Brian Greene says that it wasn't the apple that hit Newton's head, it was Newtons head that hit the apple, that the ground rushes up. By rushing up I thought he must mean the earth travelling and spinning at vast speeds crashes into the object
 
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Gravity is simply a property of matter, all matter attracts other matter, in general relativity though it can be described as geometry rather than simply as a property which matter has.
 
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  • #7
russ_watters
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Brian Greene discusses it briefly in this video from about 9.20.
When he says "When Newton was sitting there under the tree, according to Einstein, it is not that the apple fell on his head; his head rushed up and hit the apple." I can see why it might lead you to the interpretation you had, but what you said is not what he's talking about and the way he described that is not very good. He's referring to the equivalence principle, where acceleration and gravity are observed to behave the same as each other:
https://en.wikipedia.org/wiki/Equivalence_principle

It is clearer from the demonstration he gives than the description he gives.

Clearly, if we were just in the way of Earth's motion through space (he never says "traveling through space"), people on the other side of the Earth would be left behind as it moved away from them.
 
  • #8
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When he says "When Newton was sitting there under the tree, according to Einstein, it is not that the apple fell on his head; his head rushed up and hit the apple." I can see why it might lead you to the interpretation you had, but what you said is not what he's talking about and the way he described that is not very good. He's referring to the equivalence principle, where acceleration and gravity are observed to behave the same as each other:
https://en.wikipedia.org/wiki/Equivalence_principle

It is clearer from the demonstration he gives than the description he gives.

Clearly, if we were just in the way of Earth's motion through space (he never says "traveling through space"), people on the other side of the Earth would be left behind as it moved away from them.
Thanks for watching the video and providing the link, I shall have to read on because I was obviously way off.
 

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