While i don't do physics i do study mathematics, and am very interested in the field of mechanics/pure/physics. I already know that potential energy is defined by the formulae 'mgh' (mass x gravity x vertical distance). I also know that gravity marginally varies at different points along the earth. Something i was pondering was, if you moved a particle up vertically in an area of low gravity, onto a smooth plane, then moved it a relatively long distance to somewhere where gravity was much higher, then let it fall. Wouldn't you get more energy out than you originally put in? My thought here is that mh(low gravity) when raising requires less energy than the mh(high gravity) will give out when it falls. What's the problem in this train of thought?