# An apple falls on the ground. Why the Moon doesn't fall if the same force acts o

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flyskyposted 5 years ago

An apple falls on the ground. Why the Moon doesn't fall if the same force acts on both bodies?

The story on an apple and Newton is well known (and it seems that it is more than pure legend). Reportedly, when an apple had fell on Newton's head, he asked himself: "But why the Moon doesn't fall as an apple does? “ That occasion inspired him to discover the universal low of gravity.

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duffsmomposted 5 years ago

The same force does not act on both bodies. No gravity in space.

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flyskyposted 5 years agoin reply to this

Thanks duffsmom, but no... gravity doesn't need any media to act. According to the Newton's law of gravity, gravity act on both an apple and the Moon. Better to say, both of bodies interact with the Earth. The reason is in an another fact...

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Larry Fieldsposted 5 years ago

Hi flysky. You wrote:
"Why the Moon doesn't fall if the same force acts on both bodies?"

Actually, the moon does fall. The catch is that there's also a rising component to the moon's motion. To see that, imagine to a first approximation, that the moon's orbit around Earth is a polygon--with zillions of tiny sides--rather than a classical ellipse. Suppose further that this polygon is a very close fit to the ellipse.

When the moon's path finishes traversing the second half of one of the tiny sides--and just before it starts on the next minuscule 'leg' of its journey, it has gained a little vertical distance from the Earth.

Similarly, when the moon's path finishes traversing the first half of the next small side, it has lost a little vertical distance with respect to the Earth.

On balance, the 'altitude gain' due to orbital motion will be approximately equal in magnitude to the 'altitude loss' due to gravitational attraction.

I think that it's more sporting to leave the formality of integral calculus out of the explanation, don't you?

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flyskyposted 5 years agoin reply to this

Yes, surely. You gave a great explanation and I fully agree. We can say that the Moon falls as well. Only one thing to add: the key difference between the free fall and the circular motion of the Moon is the initial velocity of a body.

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flyskyposted 5 years ago

...Of course, if we throw an apple (i.e. a satellite, in reality) with velocity at least of 11.2 km/s (escape speed) it will circled around the Earth.

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