A lorry and a small car are both travelling at the same speed on a motorway. The lorry is carrying a full load of heavy goods, while the car has only the driver inside. Both drivers are fully alert and have the same reaction time. Explain why the lorry has a greater stopping distance than the car.

OCR GCSE Physics A: Gateway Science (J249) — P8 Global challenges · Explain · 4 marks · View as Markdown

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

Stopping distance is the sum of thinking distance and braking distance. Braking distance can be affected by the speed and weight of a vehicle, as well as road and tyre conditions.

Model answer (4 marks)

1. Thinking distance is the same for both vehicles because they have the same speed and reaction time.
2. The lorry has a greater weight/mass due to its full load of heavy goods.
3. Greater weight/mass gives the lorry more momentum at the same speed.
4. The higher momentum means the braking force must act for a longer time, so the lorry’s braking distance is greater, giving a greater overall stopping distance.

Examiner tips

  • Use the word ‘greater’ to show comparison; include all four points in order.
  • Show the link between mass, momentum and braking distance explicitly.

Common mistakes

  • Confusing thinking distance with braking distance; stating the lorry’s speed is higher; omitting the momentum argument.

Mark scheme (4 marks)

  1. Thinking distance is the same for both vehicles
  2. The lorry has a greater weight / mass (due to its load)
  3. Greater weight / mass means more momentum (at the same speed)
  4. Greater momentum means the braking force must act for longer, so the braking distance is greater

Key terms in this question

stopping distance

Related

More Global challenges questions

▶ Try answering this question with AI marking (free) →