Why does a spinning ice skater speed up when pulling their arms in?
Show answer & explanation
Answer: Angular momentum stays constant while pulling arms in reduces the body's rotational inertia
Angular momentum stays constant while pulling arms in reduces the body's rotational inertia ✓ — Pulling the arms inward reduces rotational inertia, so the skater spins faster while angular momentum stays nearly constant. Rotational kinetic energy does increase: the muscles do work pulling the arms inward. Adding energy and adding angular momentum are not the same thing.
The muscles generate extra rotational force when the skater tucks their arms — The skater does not gain angular momentum from an extra external twist when tucking their arms. The muscles pull the arms inward and do work, but the faster spin follows from moving mass closer to the axis while angular momentum stays nearly constant. That muscular work supplies the increase in rotational kinetic energy.
Friction between the skates and the ice drops when the arms are pulled in — Wrong. Friction change is minimal and not the cause. The speed increase comes from conservation of angular momentum: pulling the arms in reduces rotational inertia, which raises the spin rate.
More Physics in Daily Life questions
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