Momentum
Definition
How much 'oomph' a moving object carries — it depends on both mass and velocity. Heavy, fast things are hard to stop.
Symbol: · Unit: kilogram meters per second (kg·m/s)
Technical definition
The product of mass and velocity, p = mv, a vector quantity. In a closed system with no external forces, total momentum is conserved — the total before an interaction equals the total after.
Example
A 1500 kg car at 20 m/s carries 30,000 kg·m/s of momentum — the same as a 60 kg person would have at 500 m/s. That is why stopping the car takes serious brakes while stopping the person takes a hand.
Momentum is not kinetic energy, though both involve mass and velocity. Momentum (mv) is a vector and is always conserved in collisions; kinetic energy (½mv²) is a scalar and is conserved only in elastic collisions. A lump of clay sticking to a wall conserves momentum but loses kinetic energy as heat and sound — one conservation law holds, the other does not.