Also, why do you spin faster when you bring your arms in?
In: Physics
Physics of rotation is an extremely broad topic (as in, multiple college level lectures).
For your specific question, the reason we spin faster when our arms come in is that the “amount” of rotation (the technical term is angular momentum) a body is experiencing doesn’t change on its own. Just like an object will keep moving in a straight line without speeding up or slowing down if nothing is pushing on it, an object that is spinning will keep spinning at the same rate unless you do something to it.
The trick is that, for rotation, the “amount” of rotation depends not just on how fast they are rotating, but on how far from the center of rotation the stuff is. When you move your arms in, stuff is now (on average) closer to the center of rotation than before. So everything has to speed up a bit to keep that total angular momentum constant.
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