A skater spins at 2.0 rev/s with arms out. Pulling her arms in halves her moment of inertia. Predict her new spin rate, in rev/s.

Estimate on a scale from 0 rev/s (0) to 10 rev/s (10).

Hints

No external torque acts, so $I\omega$ cannot change.
$I \to I/2$ forces $\omega \to 2\omega$ to keep the product fixed.

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4 (answers within ±0.4 count)

Why

Halving $I$ doubles $\omega$ : she now spins at 4 rev/s. Neutron stars do the same trick on a cosmic scale, a collapsing stellar core spins up to hundreds of revolutions per second.

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A skater spins at 2.0 rev/s with arms out. Pulling her arms in halves her moment of inertia. Predict her new spin rate, in rev/s.

Practise Angular Momentum and Conservation

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