Classical particles

In this course, we are going to concentrate, almost exclusively, on the behaviour of non-relativistic particles of non-zero mass (e.g., electrons). In the absence of external forces, such particles, of mass $m$, energy $E$, and momentum $p$, move classically in a straight-line with velocity

$$v = \frac{p}{m},$$ (54)

and satisfy

$$E = \frac{p^2}{2\,m}.$$ (55)