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Momentum & collisions
Momentum is mass × velocity. In any collision the total momentum stays the same — even when energy doesn't.
Collision lab
Total p: 0.0 kg·m/s
p = mv · conserved every collision
The formula
p = m · v. A heavy slow truck and a light fast car can have the same momentum. Direction matters — momentum is a vector, so a ball moving left has negative p if right is positive.
Conservation of momentum
Add up the momentum before a collision. Add up the momentum after. The two totals are always equal — no matter how messy the collision.
Two kinds of collision
- Elastic — the balls bounce. Both momentum and kinetic energy are conserved. (Think pool balls.)
- Inelastic / sticky — the balls stick. Momentum is still conserved, but some kinetic energy turns into heat or sound. (Think two lumps of clay.)
Why rockets work
A rocket fires hot gas backwards. The gas gains backward momentum, so the rocket has to gain forward momentum to keep the total the same. That's how thrust works in empty space.