Chapter 14.6, Problem 2PQ

To determine

To explain:

How the fact that, $r\text{'}\text{'}$ is proportional to r in the relationship $r\text{'}\text{'}\left(t\right)=-\frac{k}{{\Vert r\left(t\right)\Vert }^2}{e}_r=-\frac{kr\left(t\right)}{{\Vert r\left(t\right)\Vert }^3}$, is used to prove Kepler’s Second Law.