A car travels at a constant speed around a circular roundabout. Explain why a net force must act on the car even though its speed does not change, and describe the direction of this force.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A car enters a roundabout and maintains a constant speed as it travels in a circular path around the central island.
Model answer (5 marks)
A car moving at constant speed around a roundabout has a velocity that is always directed tangentially to the circle. Because the direction of the velocity vector changes continuously, the car is accelerating (centripetal acceleration). According to Newton’s second law, an acceleration requires a net force. Therefore a net force must act on the car even though its speed is unchanged. This force is directed towards the centre of the roundabout, i.e. in the centripetal direction.
Examiner tips
- Use the term ‘velocity’ and note it is a vector; mention changing direction gives acceleration.
- Show that acceleration → net force via F=ma; state direction is towards the centre (centripetal).
Common mistakes
- Confusing speed with velocity and saying no force because speed is constant.
- Failing to specify the direction of the force or calling it ‘centrifugal’ instead of centripetal.
Mark scheme (5 marks)
- Velocity is a vector quantity that includes direction (not just speed)
- The direction of motion continuously changes as the car travels in a circle
- A change in direction means a change in velocity, which means the car is accelerating
- A net (resultant) force is needed to produce an acceleration / to cause a change in velocity
- The net force acts towards the centre of the circular path (centripetal direction)
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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