If you assume that being at the center of gravity is 0 energy, you'll need to expend energy to get away from that point.
Gravity isn't energy... it's just a natural state of matter. It's like pulling a spring-- the only energy comes from you pulling the spring, and the spring just tries to pull back to its natural state. Where does the spring get its energy? Spring potential, or U(s). Just like gravity-- U(g).
Just like springs, you GIVE an object potential energy by lifting it, and just like a spring, gravity just pulls it back down, converting the potential energy into kinetic energy.
If you want to follow the Zero-Energy Universe hypothesis, then gravity is a NEGATIVE energy. But I don't really understand that all that well.
Again, being stuck together is a natural state. You give an object gravitational potential energy when you lift it, all gravity does is convert that energy into kinetic energy.
To be truthful, we don't know 100% why this is-- quantum physics is the pursuit of such questions.
the potential energy is JUST A MODEL of how the universe works. All of Newtonian Mechanics is. W + U(g) + KE + U(s) = W + U(g) + KE + U(s) is just a model of the conservation of energy. We don't know for sure (yet) how gravity converts energy, but it does, and we made an equation to model that.
This post was edited by lluke9 on Nov 3 2011 05:24pm