Will Uranus keep its rotation?

Question posed by Meaghan.

On Earth, if you start a coin spinning on a table it will eventually stop. Even things like spinning tops, which are designed to keep spinning for as long as possible, will come to a halt after time. This is because a number of different forces are affecting them as they spin: friction from the surface they're spinning on, for example; air resistance for another.

These two forces always act to stop an object spinning on Earth. If you could remove them then the coin or top could potentially spin forever without stopping or even slowing down.

In space Uranus, or any other planet for that matter, does not experience such forces- there is no air in space to resist its spin; nor is Uranus resting on anything to provide a friction force. Uranus, without external forces working against it, could keep spinning indefinitely.

There are other ways forces that work against a planet's spin can be introduced. One example is a lot closer to home than Uranus:

Our own Moon is kept in orbit around the Earth by the force of gravity, but gravity works both ways: the Moon also pulls on the Earth with its own gravitational field. More importantly, it keeps hold of our oceans, pulling them towards itself- this is what causes the tides. Because the Earth spins faster than the Moon travels around it, the Moon tries to drag the oceans against the spin of the Earth. This causes a friction force to be exerted upon it. This, over billions of years, is slowing the spin of the Earth- each day is slightly longer than the last, although it won't change enough over the course of our lifetimes for us to notice this happening.


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