A force changes momentum; the longer the change takes, the smaller the force. That is how crumple zones, airbags, helmets and crash mats protect you.
What you will learn
- Use F = mΔv ÷ Δt, changing ms to s
- Find the change in momentum when something bounces back
- Explain how safety features reduce the force by increasing the time
Force is the rate of change of momentum
Newton's second law, F = ma, with a = Δv ÷ Δt, gives a second form:
force = change in momentum ÷ time taken
F = m Δv ÷ Δt
Force in N, mass in kg, change in velocity Δv in m/s, time Δt in s. AQA (Physics) and Edexcel: given on the equation sheet. You still need to rearrange it: change in momentum = F × Δt.
- A 0.43 kg ball is kicked from rest to 25 m/s in 10 ms
- 10 ms = 0.010 s
- change in momentum = 0.43 × 25 = 10.75 kg m/s
- F = 10.75 ÷ 0.010 = 1075 N
Contact times are often in milliseconds. Divide by 1000 before you use them, or the force comes out 1000 times too small.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Force is the rate of change of momentum
- Bouncing back
- Make the stop last longer
- Try it: Your first force from momentum
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Crumple zones, Seat belts, Airbags, Collapsible steering columns, Side impact bars, Headrests
In this unit: Momentum
Momentum as mass times velocity, its conservation in collisions and explosions, force as the rate of change of momentum, safety features and kinetic energy in collisions (Higher tier).
- Momentum, p = mv
- Conservation of momentum in collisions
- Explosions and recoil
- Force and the rate of change of momentum
- Collisions and kinetic energy