Mass times velocity
If mass and velocity both matter, it is natural to bring the two together into a single value.
Momentum
The momentum of an object is defined as the product of that object's mass (m) and its velocity (v).
momentum = mass × velocity = m v
The unit
The unit of mass is kg and the unit of velocity is m s−1. Since momentum is a product, the units multiply too.
The unit of momentum
unit of momentum = kg m s−1
It has no separate name of its own, the way the newton or the joule does. The two units are simply written together.
Momentum is a vector
Momentum is a vector
Because velocity is a vector, momentum is a vector quantity too. Its direction is the direction of the velocity.
Mass is a scalar; a vector multiplied by a scalar gives a vector.
As the velocity changes
| What the car does | What the momentum does |
|---|---|
| Travels fast | Its momentum is large |
| Its velocity gradually falls | The momentum falls |
| Comes to rest | The momentum becomes zero |
The mass stays the same throughout, so the momentum changes only with the velocity.