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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, m in a circular orbit with radius, r, around a body with mass M can be written
 equation7
We can eliminate v by equating the net force in circular motion to the force of gravity
equation15
Inserting this in Eq. 1 yields
equation24
Note that as we should expect for a closed orbit tex2html_wrap_inline246 is less than zero. It is this last expression for tex2html_wrap_inline246 which can be generalized to the general elliptical case:
 equation37
Where a is 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;
eqnarray43
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:
tabular52
Where tex2html_wrap_inline268 is the eccentricity of the elliptical orbit. We see that when e=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.




next up previous
Next: Speed in orbit of Up: General Physics for Bio-Science Previous: General Physics for Bio-Science

Collin Broholm
Mon Nov 3 10:13:59 EST 1997