Momentum is mass × velocity: p = m v, measured in kg m/s. It is a vector, so direction matters. In a closed system, the total momentum before an event equals the total momentum after it: momentum is conserved. When a gun fires, the bullet goes forward and the gun recoils backward with equal and opposite momentum.
Force equals the rate of change of momentum: F = m Δv ÷ Δt. Increasing the time taken to stop reduces the force. Car safety features, such as crumple zones, seat belts and airbags, all increase the time of a collision, which reduces the force on the passengers. Cycle helmets and crash mats work in the same way.
Worked example
A 2 kg trolley at 3 m/s hits and sticks to a stationary 1 kg trolley. What is their speed afterwards?
- Momentum before = 2 × 3 + 1 × 0 = 6 kg m/s.
- Momentum after = (2 + 1) × v = 6.
Answer:v = 2 m/s.
Key words
- momentum
- mass × velocity, in kg m/s
- conservation of momentum
- total momentum is the same before and after
- impulse
- change in momentum: force × time
- crumple zone
- part of a car that squashes to lengthen collision time
New questions every time.
NGSS-referenced: HS-PS2-2