Why a skater spins faster when she pulls her arms in: angular momentum and r squared
30seconds
A skater starts a slow spin with her arms wide, pulls them in, and blurs into a fast spin with no push from anything. The trick is a quantity that cannot change.
Drag her hands in and out and watch the moment of inertia, the spin rate and the energy trade off, and see why distance counts squared.
Deep dive · 4 min
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