Why do you spin faster when you tuck your arms and legs in?

Question

So far all the explanations I’ve seen have basically just said, “cause physics”.

In: Physics

Answer 1

There is something called “Conservation of angular momentum”. Essentially for spinning things, the momentum is how fast it rotates times something called the “moment of inertia”. Well, the inertia of an item is a measure of how far stuff is from the center of mass. So something spread out like a pancake, would have a bigger moment of inertia than something that got smashed down into a small ball.

So, by tucking your arms and legs in, you are reducing your “moment of inertia”. Since angular momentum is your moment of inertia times how fast you rotate, your body has to rotate faster to have the same angular momentum for your smaller moment of inertia.