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ngasnier · May 18, 2009, 12:56 PM

Hi -

I learned that any body "in orbit" is really in continuos free fall due to gravity.

But if so, why hasn't the moon crashed into the earth? An apple when dropped only takes a second to fall to the earth, surely a few million years (or whatever it is... ) is long enough for the moon.

N0help4u · May 18, 2009, 01:11 PM

For one thing, the gravity of the sun keeps the planets in their orbits.

Perito · May 18, 2009, 02:07 PM

I learned that any body "in orbit" is really in continuous free fall due to gravity.

But if so, why hasn't the moon crashed into the earth? An apple when dropped only takes a second to fall to the earth, surely a few million years (or whatever it is... ) is long enough for the moon.

If you drew a line between the earth and the moon, at any instant, the moon is moving perpendicular to that line. It is accelerating toward the center of the earth. This is the "continuous free fall" toward the center of the object. This is called centripetal (not centrifugal) acceleration. The constant acceleration toward the center of the earth coupled with the direction of motion keep it from falling into the earth. At any distance from the center of the earth, you can calculate the speed that is required for the object to be in orbit. That speed is the stable orbital speed.

Unknown008 · May 19, 2009, 11:59 AM

Another point is that if the Earth was not attracting the moon, you'd never have seen the moon you can see every night (nearly). You can see it as the moon always trying to go away, but the Earth has an invisible hand holding it, and the result is that the moon ends up revolving around the Earth.

excon · May 19, 2009, 12:13 PM

Hello n:

It IS falling towards earth. But at the same time it's being pushed away by centrifugal force... So, as it turns out, as it falls, it just misses the earth, and it's not being pushed away with enough force to make it REALLY miss the earth..

So, it's falling towards, and being pushed away from, the earth with an equal force, and that's why it stays in orbit.

excon

PS> (edited) I have no idea whether that makes any sense or not.

Unknown008 · May 19, 2009, 12:25 PM

LOL exi! In fact, an astronomer had a talk recently here in Mauritius and said that the moon is actually going away from the Earth, by 3 cm each year because the Earth is slowing down and the moon needs to get further to keep the system going.

Perhaps the world will stop when the moon will be far enough to detach itself from the gravitational force of the Earth.

Btw, centrifugal force is just an 'illusive' force. That outward force is in fact inertia.

rikesh30 · Jun 1, 2009, 10:15 AM

It stays in place because of gravity