# A stationary ice skater catches a heavy ball thrown horizontally towards them. Explain why the skater and ball move together at a lower speed than the ball had just before the catch, and identify the type of collision that has occurred.

> IB DP Physics Higher Level (2023 syllabus) — A.2 Forces and momentum · Explain · 4 marks

> An ice skater standing at rest on a frictionless ice surface catches a heavy ball moving horizontally. After the catch, the skater and ball move together as one system.

## Mark scheme (4 marks)

1. The total momentum of the system is conserved, so the momentum of ball before equals the momentum of ball plus skater after.
2. The total mass of the system increases when the skater catches the ball, since the skater's mass is added to the ball's mass.
3. Since momentum = mass × velocity, a larger mass with the same momentum means a smaller velocity / v = p/m so v decreases as m increases.
4. This is a perfectly inelastic collision because the two objects stick together and kinetic energy is not conserved / is lost.

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-stationary-ice-skater-catches-a-801560b4) · Published by Druglandscape Ltd.