Question

An ice skater can change the rate of their spin by moving their arms into and away from their body without any other forces or torques. Explain this in terms of angular momentum.

Answer 1

Angular momentum is conserved if there are no external forces

P1 = P2

I1 ω1 = I2 ω2

ω2 / ω1 = I1 / I2

If the skater pulls their arms in (I2 < I1) then the angular speed must increase for angular momentum to be conserved.