S4.8.7 For the adult
Momentum
Why do cars have parts built to crumple in a crash, when a stiff car would stay in one piece?
What this chapter teaches
Momentum is mass times velocity, in kg m/s, and it has a direction. The total momentum of objects is the same before and after a collision when no outside resultant force acts. A longer stopping time means a smaller force for the same change of momentum.
Three questions to ask
There is no one right wording. Listen for the idea.
- A 2 kg trolley moves at 3 m/s and sticks to a 1 kg trolley at rest. How would you work out their velocity?
- Why does a minus sign matter when two trolleys move towards each other?
- Why does a crumple zone make a crash safer if the change of momentum is the same?
After the chapter, the learner can
- Calculate momentum and use its conservation in collisions
- Explain how crumple zones and helmets reduce force by increasing time
What they copy into their notebook
Momentum p = m × v, in kg m/s, with a sign for direction. Total momentum is conserved in a collision. Force = change of momentum ÷ time, so a longer stop gives a smaller force.