Proof That Body in Circular Orbit Has Total Energy That is Half of Its Gravitational Potential Energy

That a body in orbit around another body has negative total energy is quite important. It allows that bound orbits are possible, applies to any system where the force maintaining the orbit is an inverse square law. The obvious examples are planetary motion and the orbit of electrons around the nucleus.

Suppose then that an object of massorbits a central body of massin a circular orbit of radiusThe (attractive) gravitational force between them isThe centripetal force iswhereis the speed of the orbiting body.

Equating these two gives(1)

The total energy of the body is the sum of the positiveand the negative

Substituting (1) into this expression gives

It is important to realise that this example only applies for circular orbits. If the orbit is not perfectly circular, then energy is constant being exchanged between gravitational and kinetic energy, so obviously the total energy, which is constant, cannot be half of the constantly changing gravitational potential energy.

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