Explain why crumple zones and seat belts are both used as safety features in modern cars to protect occupants during a collision.

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).

During a crash test, a car travelling at high speed collides with a barrier and comes to a sudden stop. Engineers analyse the forces experienced by the dummy occupants inside the car.

Model answer (4 marks)

Crumple zones are designed to deform on impact, which increases the time over which the car’s momentum changes. The longer the time, the smaller the average force on the occupants, because force equals change in momentum divided by time.
Seat belts stretch slightly when the car stops, which also increases the time over which an occupant’s momentum changes. This reduced force prevents serious injury.

Examiner tips

Use the formula F=Δp/Δt to link time and force; mention momentum change.
Show that both features act by increasing Δt, not by reducing Δp.
Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

Confusing crumple zones with airbags; they are separate systems.
Failing to mention that seat belts stretch, not just hold the occupant in place.
Using vague terms like ‘slows down’ instead of ‘increases time’.

Mark scheme (4 marks)

Crumple zones increase the time over which the momentum change occurs (during the collision)
Because the time is increased, the force experienced by the occupants is reduced
Seat belts increase the time over which the occupant's momentum change occurs (by stretching slightly)
This reduces the force experienced by the wearer, preventing serious injury

Key terms in this question

crumple zone · seat belt