Impulse: a force acting for a time
Impulse J = FΔt, its link to the change in momentum, the area under a force–time graph, and why a longer contact time means a smaller force.
Why does bending your knees on landing save your joints, and why does an airbag save a life? The same momentum has to disappear in both cases. What changes is the time.
A 0.40 kg ball hits a wall at 5.0 m s⁻¹ and stops. The impulse on it is…
A force rises uniformly from 0 to 80 N over 0.050 s, then stops. The impulse is…
The impulse in the previous question acts on a 0.50 kg trolley initially at rest. Its final speed is…
An airbag reduces injury mainly because it…
A 0.20 kg ball moving at 6.0 m s⁻¹ rebounds from a wall at 4.0 m s⁻¹. The contact lasts 0.020 s. The average force on the ball is…
Explain, using impulse, why a cricketer pulls the hands back when catching a fast ball.
F = Δp / Δt is also true when…
A ball bouncing back with the same speed has a change in momentum of…
Key points
- • Impulse J = FΔt, in N s (= kg m s⁻¹).
- • J = Δp: the applied external impulse equals the change in momentum.
- • On a force–time graph the area is the impulse; the average force is J / Δt.
- • A longer contact time gives a smaller average force for the same Δp.
Formulas
- J = FΔt = Δp
- F = Δp / Δt
Key terms
- Impulse :
- force × time of contact: equal to the change in momentum
Common errors
- • Using milliseconds without converting to seconds
- • Forgetting the sign change in a rebound
- • Treating a force–time area as a distance