# What makes the Earth keep spinning?

I am thinking of what makes the Earth keep spinning? Is there anybody here know the answer?

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Possible duplicates: physics.stackexchange.com/q/12140/2451 and links therein. –  Qmechanic Sep 25 at 5:04

I agree with the previous answer. Angular momentum, something the earth has because of its rotation about its axis, can only be changed when an external torque (twisting motion) is applied to the earth. As far as I know, there are two ways in which this can happen. If there was friction between the earth's surface and space, then that would slow down the earth. However, this is negligible because space is essentially a vacuum. The second way is through the gravitational/tidal force applied by the moon. This force creates the tidal bulge of the earth's oceans, exerting a torque. So, while the earth seems to be spinning at a constant rate, it is actually slowing down slightly over time because of these tidal forces.

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And someday the earth will stop spinning? –  Stiff Jokes Jun 22 '12 at 4:15
I believe so, though I read somewhere that before that happens, the sun will have expanded so much that it will engulf the earth. This just shows how slowly this decrease rotational speed is! –  Steven Harris Jun 22 '12 at 4:21
@Forgiver It won't ever stop spinning. If the Sun didn't turn into a red giant the Earth would eventually end up tidally locked to the moon. But as Steven pointed out the sun will eat the Earth/Moon system before that happens. –  Dan Neely Jun 22 '12 at 19:32
"And someday the earth will stop spinning?" Yes, but only relative to the Moon (in case of lunar tides). Moon has already stopped spinning relative to Earth. –  Leos Ondra Jun 30 '12 at 8:56

Spinning at a constant rate does not require any outside force or torque, due to conservation of angular momentum.

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But is there a friction force to decelerate the earth rotation? –  Stiff Jokes Jun 22 '12 at 3:51
@Forgiver: no, there isn't. –  David Z Jun 22 '12 at 3:59