Newton's second law can be written using momentum: force = change in momentum ÷ time taken. In symbols, F = (mv − mu) ÷ t, where m is mass, u is the starting velocity, v is the final velocity and t is time. The force is in newtons (N), the change in momentum is in kg m/s and the time is in seconds.
Take a 2 kg ball kicked from rest to 5 m/s. Its change in momentum is 2 × 5 − 2 × 0 = 10 kg m/s. If the kick lasts 0.1 s, the force is 10 ÷ 0.1 = 100 N. A 1000 kg car slowing from 20 m/s to rest has a change in momentum of 20 000 kg m/s. If it stops in 5 s, the force is 20 000 ÷ 5 = 4000 N.
The rule shows that for the same change in momentum, a longer time means a smaller force. Crumple zones, airbags and seat belts make a collision last longer, which reduces the force on the passengers. Catching a fast ball by moving your hands backwards does the same thing.