A lorry and a small car are both travelling at the same speed when their drivers each notice a hazard at the same moment. Both drivers are fully alert and have identical reaction times. Explain why the lorry has a greater stopping distance than the small car.

Eduqas A-Level Physics — 4.2 Kinetic theory and gases (A-Level only) · Explain · 5 marks · View as Markdown

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

The lorry has a much greater mass than the small car. Both vehicles are travelling at the same speed and both drivers react at the same time. The road and weather conditions are identical for both vehicles.

Model answer (5 marks)

1. The thinking distance is the same for both vehicles because the reaction time and speed are identical.
2. At the same speed the lorry has a greater mass, so its momentum is greater.
3. To bring the lorry to rest a larger change in momentum (Δp) is required than for the car.
4. The maximum braking force that can be applied is limited by the friction between tyres and road, so it cannot produce the larger Δp in the same distance.
5. Consequently the braking distance is greater for the lorry, making the total stopping distance (thinking + braking) greater than that of the car.

Examiner tips

  • Use the word ‘momentum’ and ‘change in momentum’ to show understanding of Newton’s second law.
  • Show the logical sequence: same thinking distance → greater momentum → larger Δp → limited braking force → longer braking distance.
  • Include the final conclusion that total stopping distance is greater for the lorry.
  • Keep each point concise and numbered to match the 5 marks.

Common mistakes

  • Confusing speed with acceleration or force; remember stopping distance depends on momentum and braking force. Failing to mention the thinking distance is the same for both vehicles. Using vague terms like ‘bigger’ without linking to momentum or force.

Mark scheme (5 marks)

  1. The thinking distance is the same for both vehicles
  2. The lorry has a greater momentum than the small car (at the same speed)
  3. A greater change in momentum is needed to stop the lorry
  4. The braking force available cannot fully compensate, so the braking distance is greater
  5. Therefore the total stopping distance (thinking distance + braking distance) is greater for the lorry

Key terms in this question

stopping distance

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