Momentum

The momentum of a mass $m$ is usually defined by

$$p = m v,$$

where $v$ is the velocity of the mass. Note that momentum is a vector quantity in general, so we should more clearly write

$$\underline{p}= m \underline{v},$$

where $\underline{p}$ and $\underline{v}$ are vectors in 3D space. We will return to vector momentum in §B.4.1 below.