What Survives a Collision
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Momentum is mass times velocity, and it is conserved in every collision.
- Two objects pushing apart gain equal and opposite momentum.
- Extending the time of a collision reduces the force felt.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
The quantity that does not change
Collisions look chaotic. Metal bends, things heat up, speeds change completely. But one quantity survives every collision unchanged, and that makes it extremely useful. It is called momentum, and it is simply mass multiplied by velocity. Add up the momentum of everything before the collision, then add it up again afterwards, and you get the same total. Energy does not behave this way — plenty of it turns into heat, sound and permanently bent metal. Momentum has nowhere else to go. It can only be passed between the objects involved.
Push off, and you both move
Picture two skaters standing still on ice, facing each other. Nothing is moving, so the total momentum is zero. Now they push against each other and slide apart. The total still has to be zero, which means their two momentums must cancel exactly: mass times velocity one way equals mass times velocity the other. If one skater is heavier, they move away more slowly, and the lighter one shoots off faster. The same arithmetic explains why a rifle kicks back into the shoulder, and why a rocket can accelerate in space with nothing to push against.
Why cars are built to crumple
A crash has to remove all of the momentum, and there is no getting around that part. What you can change is how long the removal takes. Force is the rate at which momentum changes, so stopping in a very short time means a very large force, and stopping over a longer time means a much smaller one. The total momentum removed is identical either way. This is exactly why modern cars are designed to crumple rather than to stay rigid. A car that folds takes longer to come to rest, and the people inside experience far less force. A stiffer car would survive better and its passengers would not.
Curriculum links: NGSS HS-PS2-2 · NGSS HS-PS2-3