Physics
Conservation of Angular Momentum in Figure Skating Spins
Quick fact
A figure skater can increase their spin rate by more than 100% just by pulling their arms close to their body, without any extra push.
Why this is interesting
Have you ever watched a figure skater spin faster simply by pulling their arms in? What physical principle makes this possible?
Read the full explanation
Understanding Conservation of Angular Momentum in Figure Skating Spins
Imagine a skater starting to spin with arms stretched out. Because ice and air resistance produce very little twisting force (torque), the skater's total angular momentum—the 'rotational oomph'—stays constant. Angular momentum depends on two things: how fast the skater spins (angular velocity) and how mass is distributed relative to the spin axis (moment of inertia). When the skater pulls arms in, mass moves closer to the axis, reducing moment of inertia. Since angular momentum = moment of inertia × angular velocity, a decrease in moment of inertia forces an increase in angular velocity. The skater spins faster automatically.
A deeper explanation
Conservation of angular momentum is a direct consequence of Newton's second law for rotation: net torque equals the rate of change of angular momentum. If no net external torque acts on a system, angular momentum remains constant. For a skater, the primary external torques (from ice friction and air drag) are negligible during a well-executed spin, so L = Iω is constant. This explains why pulling limbs inward (decreasing I) increases ω. The same principle governs why divers tuck to rotate faster in midair, why a spinning ice skater can slow down by extending arms, and why planets spin faster as they contract. Understanding this mechanism reveals that the skater's graceful acceleration is not magic but a precise expression of a universal physical law that governs everything from figure skating to star formation.