A skateboarder wearing a safety helmet collides with a wall and comes to rest. The helmet contains a thick layer of foam padding that compresses during the collision. Explain why wearing the helmet reduces the force experienced by the skateboarder's head during the collision.

Pearson Edexcel International GCSE Physics (4PH1) — 1.3 Forces, movement, shape and momentum · Explain · 4 marks · View as Markdown

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

During a skateboarding collision, a skateboarder's head strikes a wall. The skateboarder is wearing a helmet fitted with a thick foam padding layer. The skateboarder's head and helmet have a combined mass of 6 kg and are travelling at 4 m/s just before impact, coming to rest after the collision.

Model answer (4 marks)

The head has a change in momentum from 6 kg × 4 m s⁻¹ to 0, so Δp=24 kg m s⁻¹.
The foam padding compresses, so the collision lasts a longer time.
Force is Δp divided by time, so a longer time gives a smaller force.
Thus the helmet reduces the impact force on the head, protecting it.

Examiner tips

  • Use the impulse–momentum relation (F=Δp/Δt) explicitly. Mention the increase in collision time due to foam compression. Show the chain: momentum change → time increase → force decrease. Use the words ‘reduces the force’ and ‘protects the head’ as the scheme expects.
  • common_mistakes
  • :
  • Forgetting to state the change in momentum. Confusing force with impulse. Not linking the longer time to a smaller force. Using vague terms like ‘less damage’ instead of ‘reduced force’.

Mark scheme (4 marks)

  1. The skateboarder's head undergoes a change in momentum (from moving to stationary) during the collision
  2. The foam padding compresses, increasing the time over which the change in momentum occurs
  3. Force equals change in momentum divided by time taken, so a longer time results in a smaller force
  4. The force experienced by the skateboarder's head is therefore reduced / the helmet protects the head by reducing the impact force

Key terms in this question

force

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