Momentum is conserved in collisions — Science, 14–17 years
Momentum links an object's mass and velocity and helps predict what happens when objects collide.
What momentum means
Momentum measures how hard a moving object is to stop. It depends on both mass and velocity: a heavy lorry moving slowly can have the same momentum as a light car moving quickly. Momentum has direction, so reversing direction reverses the momentum.
Why momentum matters
A collision can make motion change very quickly, but the total momentum of a closed system stays the same. This rule lets scientists work out unknown speeds after a crash or explosion, even when the forces during the event are complicated. It comes from the equal and opposite forces objects exert on each other.
A collision calculation
A 2 kg trolley moves right at 3 m/s and hits a stationary 1 kg trolley. If they stick together, their total momentum is 2 × 3 + 1 × 0 = 6 kg·m/s. Their combined mass is 3 kg, so their shared velocity is 6 ÷ 3 = 2 m/s to the right.
The tempting mistake
A common mistake is to conserve kinetic energy as well as momentum in every collision. That seems reasonable because motion is still present afterwards, but sound, heat and bending can carry some energy away from the moving objects. Momentum is conserved in the closed system; kinetic energy is conserved only in special, elastic collisions.
Where it is used
Crash investigators use momentum to estimate vehicle speeds before a collision. Engineers also use it when designing airbags, helmets and crumple zones: these increase the time over which momentum changes, reducing the average force on a person. The calculation is useful, but real crashes involve friction and several objects.
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