A cyclist rides at a constant speed around a circular velodrome track. Explain why, even though the cyclist's speed remains constant, the cyclist is still accelerating, and describe the direction and cause of this acceleration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A velodrome is an indoor cycling arena with a circular track. A cyclist completes many laps at a steady speed.
Model answer (5 marks)
A cyclist moving at a constant speed is still accelerating because acceleration is a change in velocity, not just speed.
1. The cyclist’s velocity vector changes direction continuously as he goes round the track.
2. Since velocity changes, the cyclist must be accelerating.
3. The acceleration is directed towards the centre of the circular track (centripetal).
4. A net force towards the centre provides this acceleration.
5. That force is supplied by the friction between the tyres and the track (or the normal force from the banked surface).
1. The cyclist’s velocity vector changes direction continuously as he goes round the track.
2. Since velocity changes, the cyclist must be accelerating.
3. The acceleration is directed towards the centre of the circular track (centripetal).
4. A net force towards the centre provides this acceleration.
5. That force is supplied by the friction between the tyres and the track (or the normal force from the banked surface).
Examiner tips
- Use the definition of acceleration as a change in velocity; include the direction of the velocity vector.
- Show that the acceleration is centripetal and link it to the required net force.
- Mention the source of the force (friction or normal force).
Common mistakes
- Confusing speed with velocity; saying the cyclist is not accelerating because speed is constant.
- Forgetting to state that the acceleration is directed towards the centre of the track.
- Failing to identify the friction or normal force as the cause of the centripetal acceleration.
Mark scheme (5 marks)
- Acceleration is a change in velocity, not just a change in speed
- The direction of the cyclist's velocity is continuously changing as they go around the circular track
- Because velocity is changing, the cyclist must be accelerating
- The acceleration is directed towards the centre of the circular track
- A resultant (net) force directed towards the centre provides this acceleration; this force is the friction between the tyres and the track (or the normal contact force from the banked track surface)
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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