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This question already has an answer here:

What drives this happen? Would it be the internal energy or by an external force? I did try to Google the answer, but could not find a good one.

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marked as duplicate by John Rennie, Qmechanic Dec 26 '14 at 10:50

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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In short, it continues to rotate simply because it has no reason to stop rotating. Imagine I have a puck and I whack it across an ice-rink. The puck will continue to travel at the same speed until it hits a wall; i.e it will stay the same unless there is a reason not to. This is conservation of linear momentum as shown here.

The same thing happens with spinning objects too. Unless a spinning object has a reason to stop spinning, it will continue to spin for the rest of its life. This is known as conservation of angular momentum as shown here.

An interesting fact to note: Due to tidal acceleration, the Earth is actually slowly stopping it's rotation. This is due to, for lack of a better way of saying it, the moon giving the Earth a reason to slow down. Have no fear though, this will only continue until the rotational period of Earth matched the orbital period of the Moon, or in other words, the same side of the moon always faces the earth.

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  • $\begingroup$ Thanks for the information. Your information include why it keep on going and the resistance by tide. Does any force assist to the self rotating? $\endgroup$ – Marco Dec 26 '14 at 8:55
  • $\begingroup$ The same side of the Moon always faces the Earth already, owing to tidal locking. Another way of saying your last paragraph is the Moon's drag on the Earth is transferring kinetic energy to the Moon from the Earth's spin kinetic energy, and this process will asmyptote to the state where the Moon's orbital angular speed matches the Earth's spin angular speed. $\endgroup$ – WetSavannaAnimal Dec 26 '14 at 8:58
  • $\begingroup$ -Does any force assist to the self rotating? -No, there is no force that keeps it spinning. It's just that there is no force to oppose its spinning. (or negligible tidal force) $\endgroup$ – Joshua Lin Dec 26 '14 at 9:17

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