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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