Potential and Kinetic Energy in Orbit

There is a beautifully simple result concerning the total mechanical energy for an object in a closed orbit in a central gravitational field. The result holds for any elliptical orbit but for simplicity we shall derive the result for a circular orbit and then generalize by replacing the radius in orbit by the semi-major axis as we did when we argued for Newton's derivation of Kepler's third law.

The total mechanical energy for a planet with mass,min a circular orbit with radius,r, around a body with massMcan be written

We can eliminatevby equating the net force in circular motion to the force of gravity

Inserting this in Eq.1yields

Note that as we should expect for a closed orbitis less than zero. It is this last expression forwhich can be generalized to the general elliptical case:

Whereais the semi major axis of the elliptical orbit. So the total mechanical energy is constant and takes on similar forms for circular and elliptical orbits. In the circular orbit, since there speed is constant, we furthermore have that kinetic energy and potential energy are constants of motion. Specifically we see that;

In contrast the kinetic and potential energy in elliptical orbits are not constant but vary so that one is large when the other is small and vise versa. It is easy to derive the following equations:

Whereis the eccentricity of the elliptical orbit. We see that whene=0 corresponding to a circular orbit then we re-find the corresponding expressions listed above. The larger the eccentricity,e, the larger is variation of the potential and kinetic energies during each period of the motion.