# Apple vs. Earth - Gravity

by Flor
Tags: apple, earth, gravity, newton
 P: 9 Why does the Earth attract the apple and the apple at the same time does not attract the Earth (the Earth does not move to the apple)?
 P: 48 It does :-) An apple is so much less massive than Earth, so its effects are negligible go unnoticed. The forces of attraction will be equal and opposite. However, you must take inertia into account. The inertia of the Earth is very great, but the ineria of the apple is small. The force is strong enough to move the apple, but not strong enough to noticeably move the Earth.
 P: 9 I know, but is it observable? Or what is the best way to explain what we observe? I see an apple falling down :)
P: 48
Apple vs. Earth - Gravity

It is inertia.

Push an apple across a desk. Now try to push a car using that same force.

 I see an apple falling down :)
How do you know the Earth is not moving up towards the apple?
Math
Emeritus
Thanks
PF Gold
P: 39,491
 I see an apple falling down :)
That's because you are standing on the earth and are moving along with it. The earth does not move relative to you.

Try this experiment instead- climb up into an apple tree and, as the apple falls, let go of the tree and fall with it. You will see that the apple does not move (relative to you) while the earth is moving very rapidly toward both you and the apple!
 P: 9 Is it because the intensity of gravitational field (g) of Earth is much bigger than the intensity of gravitational field of an apple? I can give you another question - what happens if the Earth radius will expand for example 1 x 10^30 times? Then the Earth will fall on apple! Is it true? :)
 P: 48 Gravitational force doesnt depend on the radius, only mass.
 P: 48 m1 = mass of Earth m2 = mass of apple r = distance between them G = gravitational constant The magnitude of force F will be the same for both objects, only in opposite directions. Everything is the same.....except mass (ie. inertia) If the mass of the apple is increased an incredible amount, we then might observe the effects.
P: 617
 Quote by Flor Is it because the intensity of gravitational field (g) of Earth is much bigger than the intensity of gravitational field of an apple? I can give you another question - what happens if the Earth radius will expand for example 1 x 10^30 times? Then the Earth will fall on apple! Is it true? :)
No, the force is the product of both masses, and they both feel the same force (action and reaction forces equals). Think of F = m a for the apple the for the earth, where F is the same for both. For the earth, m is huge, so the acceleration a must be tiny to keep F constant and equal to the F that the apple feels.
Emeritus