A vehicle travelling at high speed is involved in a collision and comes to a sudden stop. The vehicle is fitted with a crumple zone and the driver is wearing a seat belt. Explain how these two safety features reduce the risk of serious injury to the driver during the collision.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The crumple zone and seat belt both increase the time over which the vehicle’s momentum changes.
1. The vehicle’s momentum (m×v) must change from a high value to zero during the collision.
2. Force equals the rate of change of momentum (F=Δp/Δt). A longer Δt gives a smaller force.
3. The crumple zone deforms, absorbing energy and extending the stopping time and distance.
4. The seat belt restrains the driver, making the driver decelerate with the car rather than hitting the interior, further spreading the momentum change over time.
1. The vehicle’s momentum (m×v) must change from a high value to zero during the collision.
2. Force equals the rate of change of momentum (F=Δp/Δt). A longer Δt gives a smaller force.
3. The crumple zone deforms, absorbing energy and extending the stopping time and distance.
4. The seat belt restrains the driver, making the driver decelerate with the car rather than hitting the interior, further spreading the momentum change over time.
Examiner tips
- Show the chain: momentum change → longer time → lower force; mention both features separately but link to the same principle.
- Use the exact phrase ‘rate of change of momentum’ and ‘longer time means smaller force’.
- Explain that the crumple zone works by deformation and the seat belt by restraint.
Common mistakes
- Forgetting to state that the driver’s momentum must change to zero.
- Confusing the role of the crumple zone with the seat belt (e.g., saying the belt compresses).
- Not linking the increased time to a reduced force explicitly.
Mark scheme (5 marks)
- Both the crumple zone and seat belt increase the time over which the momentum change occurs (during the collision)
- The driver's momentum must change (from a high value to zero) during the collision
- Force is equal to the rate of change of momentum, so a longer time means a smaller force
- The crumple zone compresses during the collision, increasing the stopping time/distance for the vehicle
- The seat belt holds the driver back so they decelerate with the car rather than hitting the interior, further spreading the momentum change over time
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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