A car travels at constant speed around a circular roundabout. Explain why the car is accelerating even though its speed is not changing, and describe the direction and cause of the resultant force that produces this acceleration.

Eduqas A-Level Physics — 1.5 Circular motion (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. Velocity is a vector, so it has both magnitude (speed) and direction.
2. While the car’s speed is constant, its direction changes continuously as it goes round the roundabout.
3. A change in velocity (even if the speed is unchanged) is an acceleration.
4. The resultant force that causes this centripetal acceleration acts toward the centre of the circle.
5. This force is supplied by the friction between the car’s tyres and the road surface.

Examiner tips

  • Use the term ‘centripetal force’ and state it acts toward the centre.
  • Show that acceleration is a change in velocity, not just speed.
  • Mention friction as the source of the force.
  • Keep each point brief and to the point.

Common mistakes

  • Confusing speed with velocity; not recognising the vector nature of velocity.
  • Forgetting that acceleration can occur with constant speed.
  • Failing to identify the direction of the centripetal force.

Mark scheme (5 marks)

  1. Velocity is a vector quantity, having both magnitude (speed) and direction
  2. The direction of the car's motion is continuously changing as it goes around the roundabout
  3. A change in velocity (even at constant speed) means the car is accelerating
  4. The resultant force (centripetal force) acts towards the centre of the roundabout / circle
  5. This resultant force is provided by friction between the car's tyres and the road surface

Key terms in this question

acceleration · resultant force

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