Why do skaters spin faster with their arms in? Angular momentum, explained

Transcript

Why do skaters spin faster with their arms in? Nothing pushes them. Once a skater is spinning, there's no outside twist to speed them up. What they have is angular momentum: their total amount of spin. On smooth ice, very little can take it away. Angular momentum depends on two things: how fast you turn, and how far your mass sits from the middle. Mass far out counts for much more. Twice as far counts four times as much. Arms wide, this skater turns about one and a half times a second. Now pull the arms in. With less mass far from the axis, the same spin has to show up as speed. Arms hugged tight, she speeds up to about four turns a second, and the best skaters reach five. Push them back out, and the spin slows right down, without any brake. And here's a twist: the spin energy actually goes up as the arms come in. That energy comes from the skater's muscles, pulling inward against the spin. You can feel it yourself on an office chair, holding two heavy books. Try it yourself at Explainer Tools dot com.