A car is travelling at high speed when the driver suddenly sees a hazard ahead. The car has a seat belt fitted as a safety feature. Explain why wearing a seat belt reduces the risk of serious injury to the driver during the collision that follows.

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).

A driver is travelling along a motorway at high speed. The driver notices a hazard and reacts, but the car collides with the hazard before it can stop.

Model answer (5 marks)

The driver has momentum because of their mass and the car’s velocity. When the car collides with the hazard the driver’s momentum must change to zero. The seat belt applies a force over a longer time, so the rate of change of momentum (Δp/Δt) is smaller. A smaller force means the driver is less likely to suffer serious injury.

Examiner tips

  • Use the term ‘momentum’ and ‘force = Δp/Δt’
  • Show the chain: momentum → change → time → force → injury risk
  • Keep the answer concise and directly linked to the marks

Common mistakes

  • Confusing force with acceleration; not mentioning the time factor
  • Using vague phrases like ‘slows down’ instead of ‘changes momentum’
  • Omitting the link between reduced force and lower injury risk

Mark scheme (5 marks)

  1. The driver has momentum (due to their mass and velocity) before the collision
  2. During the collision the driver's momentum must change (to zero / decreases to zero)
  3. The seat belt increases the time over which the momentum change occurs
  4. Force is equal to the rate of change of momentum, so a longer time means a smaller force
  5. Therefore the force experienced by the driver is reduced, lowering the risk of injury

Key terms in this question

seat belt

Related

More Kinetic theory and gases (A-Level only) questions

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