4Spooled, maybe you're winding us up here...an object that has been sitting on top of a hill certainly does have an amount of potential energy; when it starts rolling down that potential is converted to kinetic energy, but when it comes to rest at the bottom it doesn't magically get that potential energy back again!
Imagine a frictionless rollercoaster; sitting still at the top of the track, it has no kinetic energy but a heap of potential. Given a tiny push to get going, it will accelerate down the track, converting that potential into kinetic energy.
As it goes up the next hill, it will convert kinetic back into potential, slowing down as it goes up.
In an ideal world, it would be able to just get up a hill of equal height to the start point as it came to a halt, but in the real case it won't ever make it back to the same height because of frictional losses of energy from the wheels rubbing on the track, bearings rubbing, air resistance and so on.
To clear things up, borrow a high school physics text from the library and start working through it; it'll certainly help with these concepts, particularly if you are thinking of instructing at any stage - at the moment, you're shooting from the hip, by the looks.