Short · Motion
Why do skaters spin faster with their arms in?
No outside twist speeds a skater up. Pulling the arms in lowers the moment of inertia, so to keep the same angular momentum, the body has to turn faster.
Try it yourself
Opens Spin Lab. Change the same thing yourself.
Transcript
Why do skaters spin faster with their arms in? Nothing pushes them. No outside twist speeds them up. A spinning body keeps its total spin, called angular momentum. That depends on two things: how fast it turns, and how far its mass sits from the middle. Now pull the arms in. With less mass far out, the body has to turn faster to keep the same spin. Arms out, and it slows right down. Try it yourself at Explainer Tools dot com.