# Explain why a spinning ice skater rotates faster when they pull their arms close to their body, and outline what happens to the skater's rotational kinetic energy during this process.

> IB DP Physics Higher Level (2023 syllabus) — A.4 Rigid body mechanics (HL only) · Explain · 4 marks

> An ice skater begins a spin with arms extended outward. They then pull their arms tightly against their body, causing their rate of spin to increase noticeably.

## Mark scheme (4 marks)

1. No net external torque acts on the skater (friction at the blade contact point is negligible), so angular momentum is conserved.
2. Pulling arms inward reduces the skater's moment of inertia (mass is redistributed closer to the rotation axis).
3. Because L = Iω is conserved and I decreases, the angular velocity ω must increase.
4. The rotational kinetic energy increases because the skater does work (uses internal/muscular energy) pulling their arms inward against the centrifugal effect, so rotational KE is not conserved.

## Key terms

- [rotational kinetic energy](https://www.gradenine.co.uk/glossary/rotational-kinetic-energy)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-spinning-ice-skater-d284f104) · Published by Druglandscape Ltd.