Brainlag PlusA force changes momentum; the longer the change takes, the smaller the force. That is how crumple zones, airbags, helmets and crash mats protect you.
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 in Plus.
Screen 1 of 4, free to read.
- 10 exam-style questions, marked as you go, each with a worked solution
- Anything you get wrong is saved to your flashcards