MOMENTUM
2 marks · EasyExplain, using physics ideas about momentum, why soft sand at the end of an escape lane reduces the force on a lorry during an emergency stop.
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Soft sand increases the time taken to stop the lorry. For the same change in momentum, increasing the stopping time reduces the force, as force = change in momentum / time.
Also accept: The momentum changes over a longer time, so force is smaller.; Increasing stopping time for the same momentum change reduces force.
Common mistakes: Talking about energy instead of momentum; Saying sand 'absorbs the force' without linking to time
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Marking points
B1 · 1 mark · States that stopping time is increased by soft sand.
B2 · 1 mark · Links increase in time to decrease in force using the idea of force = change in momentum / time.
Worked solution
Soft sand increases the time over which the lorry comes to a stop. Since the force equals the change in momentum divided by the time, a bigger time means the force is smaller for the same momentum change.