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Originally Posted by cnewtongifford
WHERE DOES THE ENERGY COME FROM WHICH CAUSES EARTH TO ROTATE?
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Stuff fell closer to the center of mass of the cloud of debris the Earth was built from. So it started rotating faster. This is analogous to the old experiment of turning on a rotating office chair with bricks in your hands. If you pull the bricks closer, you'll spin faster.
What's causing the Earth to spin is inertia. And the question shouldn't be what's causing the Earth to spin - rather, why doesn't it stop spinning? Friction is slowing it down, but the only friction of any importance is gravitational friction from the moon. And it is indeed slowing the Earth down - but very slowly. The moon got it bad, though; that's why it keeps showing the same face to the Earth. Eventually, both the Earth and the moon will show the same faces to each other due to gravity.
