# General Potential Gravitational Energy

1. Mar 30, 2006

### jubaitca

I was just wondering if there were any applications of the equations
U= GMm/r. Which basically means we are not to involve anything on earths surface.

Thx

2. Mar 31, 2006

### Staff: Mentor

Sure. Whenever you need the gravitational PE between two objects. For example, you can use this to calculate the velocity that an object needs to have so that it doesn't fall back down to earth (so-called "escape velocity").

For objects that stay near the earth's surface, it's more convenient to use U = mgy to study changes in gravitational PE. But if the object gets too far from the surface, the value of "g" decreases, and that simple equation no longer works accurately. That's when you need to use U = -GMm/r.

Note that the reference point where U = 0 is different for each formula. (The reference point is arbitrary, since only changes in U matter.) When using U = mgy, the reference point is some position where y = 0, chosen as convenient for the problem that you are trying to solve; when using U = -GMm/r, the reference point is at r = infinity.

3. Apr 1, 2006

### pmb_phy

Sure there are. See the example I layed out at

http://www.geocities.com/physics_world/gr/red_shift.htm

What you're looking for is at the bottom of the page.

Pete