What momentum actually measures
Momentum combines how much mass something has with how fast it’s moving:
p = mv
A heavy truck moving slowly and a light motorcycle moving fast can have similar momentum — both are hard to stop, just for different reasons. Momentum is a useful quantity specifically because, unlike speed alone, it captures how much “effort” it would take to change an object’s motion.
The big rule: momentum is conserved
In any collision — a car crash, two pool balls hitting each other, a rocket ejecting fuel — the total momentum of everything involved right before the collision equals the total momentum right after, as long as no outside force interferes. This is the law of conservation of momentum, and it holds true regardless of what happens during the collision itself, even if it’s violent or the objects stick together.
Two very different kinds of collisions
Elastic collisions — objects bounce off each other, and kinetic energy (energy of motion) is conserved along with momentum. Billiard balls colliding are a close real-world approximation.
Inelastic collisions — objects deform, generate heat or sound, or stick together, and kinetic energy is not conserved (some converts to other forms of energy) — but momentum still is. A car crash is a classic (unfortunately) inelastic collision.
Why momentum, and not just force, matters here
During a collision, forces can be enormous but only act for a tiny fraction of a second, making them hard to measure directly. Momentum sidesteps that problem entirely — you only need to know how fast things were moving before and after, not the complicated details of the collision itself.
Why this matters
Conservation of momentum is why airbags and crumple zones work: they don’t change how much momentum has to be absorbed in a crash, but they stretch out the time it takes to absorb it, which reduces the force experienced by passengers. It’s also exactly how rockets work in the vacuum of space, where there’s nothing to push against except their own exhaust.