# A ball on a string is swung in a horizontal circle at a constant speed. Explain why, even though the ball moves at constant speed, it is still accelerating, and describe the direction of the force that causes this acceleration.

> WJEC A-Level Physics (Wales) — 1.5 Circular motion (A-Level only) · Explain · 5 marks

> A student claims that because the ball moves at constant speed around the circle, no resultant force can be acting on it. The student is incorrect.

## Mark scheme (5 marks)

1. Velocity is a vector quantity, having both magnitude (speed) and direction
2. The direction of the ball's motion is constantly changing as it moves around the circle
3. A change in velocity (even with constant speed) means the ball is accelerating
4. A resultant force is needed to cause this acceleration (by Newton's second law / F = ma)
5. This force (centripetal force) acts towards the centre of the circle at all times

## Key terms

- [acceleration](https://www.gradenine.co.uk/glossary/acceleration)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-ball-on-a-string-is-c4319fa1) · Published by Druglandscape Ltd.