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Q & A: earth and apple

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Most recent answer: 10/22/2007
Q:
Is the only reason why an apple falls downward to meet the earth instead of the earth rising upward to meet the apple is that the eath is much more massive and so exerts a much greater pull?
- Shannon Lee (age 19)
Enverga University, Philippines
A:
If you try to use a standard 'pull' picture with Newtonian forces, which is what I think you have in mind, the Earth and the apple exert exactly the same pull on each other. The apple has less mass, so that same pull causes it to accelerate a lot more.

If you try to look at it in a General Relativistic way, neither one pulls on the other. Each distorts space-time in its vicinity. The Earth makes a much bigger distortion because it's much bigger. So if you want to think of that distortions as caused by a sort of 'pull' on spacetime, you could say the Earth pulls on it a lot more.

Mike W.

(published on 10/22/2007)

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