Momentum

Momentum is mass times velocity. In a collision or explosion, total momentum is conserved (the same before and after), as long as no outside force acts.

Key facts

  • p = mv (kg m/s). It has direction.
  • Total momentum before = total momentum after.
  • Force = rate of change of momentum: F = Δp/Δt.
  • Impulse = FΔt = change in momentum. Crumple zones and airbags increase Δt, so the force is smaller.

Worked examples

Example 1

A 2 kg trolley at 3 m/s hits a still 1 kg trolley and they move off together. Find their speed.

  1. Before: 2 × 3 + 1 × 0 = 6 kg m/s.
  2. After: 3v = 6 v = 2 m/s

Answer: 2 m/s

Example 2

A 0.15 kg ball at 20 m/s is stopped by a catch in 0.1 s. Find the average force.

  1. Δp = 0.15 × 20 = 3 kg m/s.
  2. F = 3 ÷ 0.1 = 30 N

Answer: 30 N