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

In: Physics

Energy and momentum are always conserved, so if momentum changes in something it has to come from or go somewhere else. This statement is where I’ll draw the “cause physics” line as discussing why that is true gets into above ELI5 territory, and is at the limit of my understanding.

Now we can discuss how conservation of momentum affects your question about spinning. Anything that spins has a particular amount of angular momentum. The amount of momentum is a combination of how fast it spins, how much mass it has, and how far away from the center (axis of rotation) of the spin that mass is (further out = more momentum). For a figure skater their total mass doesn’t change. If they have their arms or legs out then that mass is farther out from their axis of rotation. When they bring their limbs in their mass is concentrated near the axis of rotation. For momentum to be conserved they have to be spinning slower with their limbs out and faster with their limbs in.

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Arms and legs out = big circle

Arms and legs in = small circle

If you have the same oomph, but less distance to move, you move faster.

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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.