A roller-coaster carriage travels at constant speed around a circular loop. Explain why the carriage is accelerating even though its speed does not change, and describe the direction and cause of this acceleration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A roller-coaster carriage moves at a constant speed of 12 m/s around a vertical circular loop. The carriage completes the loop without stopping.
Model answer (5 marks)
The carriage is accelerating because velocity is a vector; its direction changes continuously around the loop even though its speed is constant. A change in direction is a change in velocity, which is an acceleration. This acceleration is directed towards the centre of the loop – the centripetal acceleration. It is produced by a net force acting towards the centre, supplied by the track (normal force) and gravity acting on the carriage.
Examiner tips
- Use the word ‘velocity’ to show vector nature; mention speed is constant but direction changes.
- Explain that change in direction = change in velocity = acceleration.
- State that the acceleration is centripetal, i.e. towards the centre.
- Mention the forces (track/normal and gravity) that provide the centripetal force.
Common mistakes
- Saying the carriage is not accelerating because speed is constant; ignoring the vector nature of velocity.
- Failing to specify the direction of the acceleration (centripetal).
- Not identifying the forces that supply the centripetal force.
Mark scheme (5 marks)
- Velocity is a vector quantity, so it has both magnitude (speed) and direction
- The direction of motion is continuously changing as the carriage travels around the loop
- A change in direction means a change in velocity, and a change in velocity is an acceleration
- The acceleration is directed towards the centre of the circular loop (centripetal direction)
- This acceleration is caused by a resultant force acting towards the centre of the loop (provided by the track pushing on the carriage / gravity / the normal contact force)
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
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- Decode the mark scheme abbreviations →
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