Define impulse and show that it equals the change in momentum. Explain why a cricket player draws the hands back while catching a ball.

1. 1 · Define impulse. Impulse is the product of a force and the time for which it acts: J = F Δt. It is a vector; unit N s.
2. 2 · Use the second law. For a body whose velocity changes from u to v in time Δt, F = (mv − mu)/Δt.
3. 3 · Multiply by Δt. F Δt = mv − mu = Δp. So impulse equals the change in momentum.
4. 4 · Apply it to the catch. The ball's change in momentum is fixed, from mu to zero. Drawing the hands back makes Δt longer.
5. 5 · Draw the conclusion. Since F = Δp/Δt, a longer time gives a smaller force, so the hands are not hurt.
6. 6 · Give another example. Airbags and crumple zones in cars work the same way.
7. 7 · Close in one line. The same change in momentum over a longer time needs a smaller force.

Scoring tip: Say which quantity stays fixed (the change in momentum) and which one changes (the time). That one line earns the reasoning mark.