Why does kinetic energy equal negative half potential energy in gravity?

[itachipower]

So today in class we were talking about gravity and my professor said that generally KE = -.5PE
And so TME = .5PE

I don't understand how that works.

 

[hilbert2]

That's the virial theorem, and it holds in that form for interacting masses whose potential energy is proportional to the inverse of their separation (e.g. gravitational interaction). Note that the kinetic energy and potential energy appearing in that equation are averages over time.

http://en.wikipedia.org/wiki/Virial_theorem

 

[itachipower]

Oh okay! Thanks!