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Old Jul 17th 2017, 09:08 AM   #1
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Why does the moon not fall into the Earth?

I know that the gravitational pull of the Earth on the moon makes moon orbit the Earth. But the reason why moon does not fall off in a straight line towards the Earth is that the moon has orbital angular momentum.

But how does the moon having orbital angular momentum prevent it from falling towards the Earth?
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Old Jul 17th 2017, 09:15 AM   #2
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Some understanding.

Angular momentum is what keeps the Earth in its stable orbit around the Sun and the Moon in a stable orbit around the Earth. It is the outward force an object exerts as it is spun around. The outward force of the Moon's angular momentum and the inward pulling force of the Earth's gravity balance each other out to keep the Moon in a stable orbit.

If the Moon were to suddenly stop rotating around the Earth, our planet would pull it straight in. The same would happen to Earth if it had suddenly stopped in its orbit around the Sun. Since an apple has no angular momentum relative to the Earth, the Earth pulls it straight down as a result.

But why does the moon have angular momentum at the first place? This is another question.
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Old Jul 17th 2017, 09:19 AM   #3
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Originally Posted by avito009 View Post
I know that the gravitational pull of the Earth on the moon makes moon orbit the Earth. But the reason why moon does not fall off in a straight line towards the Earth is that the moon has orbital angular momentum.

But how does the moon having orbital angular momentum prevent it from falling towards the Earth?
The fact in evidence is that the moon has a certain speed and direction at any one time. The pull of the Earth causes the moon to fall. However as the moon falls, so does the surface of the earth, i.e. the surface drops away at the exact same rate that the moon falls. So its said that the moon falls around the earth. From that we know that the moon moves in approximately a circle around the Earth. Using that information and the mass of the moon we calculate the angular momentum of the moon and we can show that this is a conserved quantity. That is not what is used to prove that the moon is in orbit. It's merely a result of it.

Newton is the one who first showed how objects can orbit the earth by imagining a cannon on the top of a mountain. The faster the cannon balls move the farther they travel before hitting the ground. But the further they move the more the surface of the earth curves away.

If you'd like to see his exact argument and the picture he drew go to:
https://en.wikipedia.org/wiki/Newton%27s_cannonball
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