Momentum and safety features revision notes

Edexcel International GCSE Physics (4PH1), chapter 4: Momentum, specification point 1.26P, 1.28P. Free lesson.

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Safety features reduce injury by increasing the time taken for a person or vehicle to stop. The change in momentum stays the same, so the longer stopping time gives a smaller force.

How safety features reduce injury

In a crash, a person goes from moving to stopped. That means there is a change in momentum, whatever brings them to rest. A safety feature makes the stopping take longer. Since force is the change in momentum divided by the time taken, the same change over a longer time means a smaller force on the body, and therefore less injury. Force = change in momentum ÷ time taken The full chain is: moving to stopped, longer time, smaller force, less injury.

How each feature makes the stop take longer

Each feature changes how the person or vehicle stops. It does not prevent the stop. Instead, it makes the stopping happen more gradually by stretching, squashing, crumpling or bending. Crumple zone: without it, the car body stops almost at once. The front crumples, so the car takes longer to stop. Seat belt: without it, you keep moving and hit the dashboard. The belt stretches slightly, so you slow down more gradually. Air bag: without it, your head hits the steering wheel. The bag squashes, so your head stops over a longer time. Cycle helmet: without it, your head hits the…

Comparing the force with and without an air bag

A 70 kg passenger travelling at 15 m/s has the same change in momentum whether she stops against an air bag or the steering wheel. The air bag increases the stopping time, so the force is smaller.

How to explain the effect in an exam

For a full explanation, state what the feature does to the time, what stays the same, and what happens to the force and injury. Naming the same change in momentum is important because the person or car still stops. For a crumple zone: it collapses, increasing the time taken for the car to stop. The change in momentum of the car and the people is the same. Force = change in momentum ÷ time, so the force is smaller. A smaller force acts on the people, so they are less injured.

Applying the chain to a padded mat

A gymnast lands on a thick padded mat instead of a hard floor. The mat squashes, increasing the time the gymnast takes to stop. Her change in momentum is the same, so the force on her is smaller and she is less likely to be injured.