# Gravitational Potential Energy - Orbits

## Homework Statement

A satellite is in a circular orbit 655km above Earth's surface. Determine the magnitude of the gravitational acceleration at this height.

Eg=mgh
Eg=-GMm/r

## The Attempt at a Solution

Would I just set the aforementioned eqations equal to each other and re-arrange to solve for g? I would also have to solve for the change in Eg, as well, right?

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Doc Al
Mentor
Would I just set the aforementioned eqations equal to each other and re-arrange to solve for g?
No. Instead of potential energy, think in terms of forces and acceleration. What is the gravitational force on the satellite. (Tip: Don't be so quick to plug in numbers.)

Well, it's also experiencing a centripetal force. If we set Fc=Fg, we can get the velocity of the orbit, but, other than that, I'm not sure how to do this without the mass of the sat.

Doc Al
Mentor
Well, it's also experiencing a centripetal force. If we set Fc=Fg, we can get the velocity of the orbit, but, other than that, I'm not sure how to do this without the mass of the sat.
You won't need the mass--just call it 'm'. When you calculate the acceleration, it will drop out.

What is Newton's law of Universal Gravity?

Got it - I was overthinking a rather simple question..., thanks.

Also, E=mgh only works for constant gravitational fields. i.e. at the surface of a very large mass (like the Earth).