Explain why headrests fitted to car seats reduce the risk of serious neck injury during a rear-end collision.

Eduqas GCSE Physics — 4.3 Safety in public transport · Explain · 4 marks · View as Markdown

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

When a car is struck from behind, the car and seat move forward suddenly but the passenger's head initially stays behind before snapping backwards.

Model answer (4 marks)

In a rear‑end collision the car and seat accelerate forward, but the passenger’s head, due to inertia, initially remains in its original position. This creates a relative motion where the head snaps backwards against the body, producing a whiplash injury to the neck.

A headrest fitted behind the seat provides a physical barrier that limits how far the head can move backwards. By stopping the head earlier it increases the time over which the head is brought to the same forward speed as the body, thereby reducing the acceleration (and force) on the neck.

Examiner tips

  • Use the word ‘inertia’ to explain why the head lags behind. Show the causal chain: car moves → head stays → snap backwards → whiplash. Mention the headrest’s role as a barrier and as a means of increasing acceleration time.
  • common_mistakes
  • :
  • Forgetting to mention inertia or the head staying stationary. Failing to explain that the headrest increases the time of acceleration, not just stops the head. Using vague terms like ‘helps’ without specifying how it reduces force.

Mark scheme (4 marks)

  1. In a rear-end collision the car and seat accelerate forward but the passenger's head tends to remain stationary (due to inertia)
  2. This causes the head to snap/jerk backwards relative to the body, which can cause a whiplash injury to the neck
  3. The headrest stops the head from moving too far backwards by providing a support/barrier behind the head
  4. The headrest extends the time over which the head accelerates forward (with the body), reducing the force on the neck

Key terms in this question

headrest · rear-end collision

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