Momentum and collisions — Physical education, 14–17 years
Momentum helps us describe how difficult it is to stop a moving object. It connects mass and velocity, so it is especially useful when objects collide.
The idea
A fast, heavy object is harder to stop than a slow, light one. Momentum measures this moving “push”: momentum equals mass multiplied by velocity, written p = mv. It has direction, so reversing the motion reverses the momentum.
Why it matters
After a collision, objects may move differently, but the total momentum of an isolated system stays the same. This rule solves a practical problem: it lets us work out unknown speeds without following every complicated force during the impact.
A worked collision
A 2 kg trolley moves east at 3 m/s, while a 1 kg trolley is still. Their total momentum is (2 × 3) + (1 × 0) = 6 kg·m/s east. If they stick together, their combined mass is 3 kg, so their speed is 6 ÷ 3 = 2 m/s east.
The common trap
A common mistake is to add speeds instead of signed momenta. That seems reasonable because speed is the quantity we notice, but opposite directions matter: 5 kg·m/s east and 5 kg·m/s west give a total of zero, not 10.
Outside school
Crash investigators use momentum ideas to estimate vehicles’ speeds before a collision. Sports equipment uses the same thinking: a tennis racket changes a ball’s momentum, while padding increases the stopping time and can reduce the force on a person.
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