# 21.3: Inverse Square Attractive Force

This is dealt with in detail in Chapter 9 of Celestial Mechanics. Here we investigate some general properties of the motion.

If \( F = - \frac{GMn}{r^2} \) then \( V = \frac{GMn}{r} \), and hence

\[V' = - \dfrac{GMn}{r} + \dfrac{L^2}{2mr^2}. \tag{21.3.1}\label{eq:21.3.1}\]

I sketch this in Figure XXI.1. The total energy (potential + kinetic) is constant (independent of \(r\)) and is greater than (or equal to) the potential energy. If the total energy is less than zero, you can see from the graph that \(r\) has a lower (perihelion) and upper (aphelion) limit; this corresponds to an elliptic orbit. But if the total energy is positive, \(r\) has a lower limit, but no upper limit; this corresponds to a hyperbolic orbit. If the total energy is equal to the minimum of \(V'\), only one value of \(r\) is possible, and the orbit is a circle.