A figure skater spins with angular velocity omega_1 with arms outstretched. When she draws her arms inwards, her moment of inertia decreases by 40%. What happens to her rotational kinetic energy?

Answer and explanation

Correct answer: C. It increases by 66.7%

  1. Step 1: Angular momentum L = I omega is conserved, so omega_2 = omega_1 / 0.6 = 1.667 omega_1. Step 2. Rotational energy E = L^2 / (2I). Since I drops to 0.6I, E_2 / E_1 = 1 / 0.6 = 1.667 (+66.7%). Muscular work supplies this energy. Answer: It increases by 66.7%. Why not the others:* Kinetic energy is not conserved because internal work is done by the skater.

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