# Gravitational forces

1. Apr 2, 2005

### bmoran08

When in orbit, a communication satellite attracts the earth with a force of F and the earth-satellite gravitational potential energy (relative to zero at infinite separation) is - U. The radius of earth is assumed to be r.

How do I find the satellite's altitude above the earth's surface and the mass of the satellite?

2. Apr 2, 2005

### quasar987

Obtain an expression for r²/m from the gravitation law of force between the earth and your satellite, and obtain an expression for r/m from the potential energy of the system. Plug the two expressions together.

I found r = -U/F and m = U²/FGM

(meaning $r= \nabla F/F$.. kinda cool.)

Last edited: Apr 2, 2005