A figure skater is spinning with arms extended. When she pulls her arms in, which of the following quantities increases?

Core Concept

Angular momentum is conserved because there is no external torque. When the skater pulls her arms in, her moment of inertia decreases. Since L = Iω and L is constant, ω must increase. Rotational kinetic energy K = L²/(2I) increases because I decreases while L remains constant.