Physics
Conservation of Angular Momentum in Figure Skating
Quick fact
A skater can increase their spin rate by more than three times simply by drawing in their arms and legs, demonstrating a fundamental law of physics.
Why this is interesting
Have you ever wondered why a figure skater spins faster the moment they pull their arms close to their body?
Read the full explanation
Understanding Conservation of Angular Momentum in Figure Skating
Imagine a skater spinning on the ice with arms stretched out. Their body has a certain ‘rotational laziness’ called moment of inertia – the more mass is far from the spin axis, the higher it is. When they pull their arms in, they reduce that laziness. But here's the key: if no external force (torque) acts on them, their total spin energy – angular momentum – stays the same. Since angular momentum equals moment of inertia times spin speed, when the first number drops, the second must rise. That’s why they speed up. It’s like a dancer who pulls in their arms to whirl faster, or an ice skater who finishes a spin with breathtaking speed.
A deeper explanation
Angular momentum (L) is a conserved quantity in a closed system with no external torque. For a figure skater, the ice exerts minimal friction (negligible torque) so L is nearly constant. L = I × ω, where I is moment of inertia and ω is angular velocity. Moment of inertia depends on how mass is distributed relative to the axis of rotation – arms extended increase I, arms pulled in decrease I. Because L cannot change, a decrease in I forces an increase in ω. This principle applies to all rotating bodies, from planets to molecules. In figure skating, it allows skaters to control spin speed dramatically, and understanding it helps explain why tightening body shape leads to faster spins – a perfect interplay of physics and athletic grace.