Why is gravitational potential energy relative to the height you set?

1. Apr 10, 2012

baouba

Why is gravitational potential energy relative to the height you set?

When I say relative, I don't mean anything having to do with relativity. Obviously as you go further from the earth, you have more potential energy but in the equation Ug=mgh if you set the height to 0 there is no potential energy. If you set h to 5 say, for example, you get potential energy even if you're talking about the same point in space.

This makes no sense that you can assign any height to a point on earth and change an object's energy. I just don't get how it can have "this" much potential energy when I can easily change the reference height and also change the energy.