Momentum

A moving object has momentum because it has mass and velocity. An object with greater mass or greater speed has a greater magnitude of momentum.

Momentum is the product of mass and velocity:

Here, $$p$$ is momentum, $$m$$ is mass in kilograms and $$v$$ is velocity in metres per second. The SI unit of momentum is kilogram metre per second, written $$\mathrm{kg,m/s}$$.

Momentum is a vector quantity: it has both magnitude and direction. Its direction is the same as the object's velocity. For example, an object of mass $$2\ \mathrm{kg}$$ moving east at $$3\ \mathrm{m/s}$$ has momentum $$6\ \mathrm{kg,m/s}$$ east.

Rearranging the momentum equation

If momentum and velocity are known, mass can be found by dividing momentum by velocity. If momentum and mass are known, velocity can be found by dividing momentum by mass:

When calculating along a straight line, a direction can be chosen as positive. Momentum in the opposite direction then has a negative value, as does velocity in that direction. Finding mass using $$m = p/v$$ requires a non-zero velocity.