How the force changes during a collision: the force rises to a peak and falls back to zero. A safety feature makes the collision last longer, so the peak force is lower.
Worked example 1: A constant force of 200 N acts on a trolley for 1.5 s.
- Change in momentum = F × t
- Change in momentum = 200 × 1.5 = 300 kg m/s
Worked example 2: A driver of mass 60 kg is travelling at 12 m/s and is brought to rest.
- Change in momentum = mv − mu = 0 − (60 × 12) = −720 kg m/s (a size of 720 kg m/s)
- Stopped in 0.8 s (seat belt and airbag): F = 720 ÷ 0.8 = 900 N
- Stopped in 0.1 s (hitting a hard surface): F = 720 ÷ 0.1 = 7200 N
- Same change in momentum, but a longer time gives a much smaller force
Worked example 3: A cricket ball of mass 0.15 kg travelling at 40 m/s is caught and brought to rest in 0.003 s.
- Change in momentum = mv − mu = 0 − (0.15 × 40) = −6 kg m/s (a size of 6 kg m/s)
- F = Δp ÷ t = 6 ÷ 0.003 = 2000 N
- Catching with "giving" hands makes the time longer, so the force is smaller
Quick Check: A ball hits a wall with a momentum of 8 kg m/s and bounces back with a momentum of 8 kg m/s in the opposite direction. What is the change in momentum?
Take the original direction as positive. Change in momentum = final momentum − initial momentum = (−8) − (+8) = −16 kg m/s, so the size of the change is 16 kg m/s. The minus sign shows the direction has reversed. This is why a ball that bounces back needs more force than one that is stopped dead.
This key facts covers Force During a Collision: Worked Examples within Force & Collisions for GCSE Physics. Revise Force & Collisions in Forces for GCSE Physics with 10 exam-style questions and 10 flashcards. Use this page as part of a wider topic revision path rather than treating it as an isolated fact. It is section 3 of 7 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Force & Collisions
Which equation links force, change in momentum and time?
Explain how an airbag reduces the risk of injury to a driver in a collision.