Hi everyone, First, sorry if the question is stupid, I do not have any background in Physics. Just very curious. Potential energy due to gravity is: m * g * h Joules Now, suppose I lift a weight on Moon at a given height. I will have: 1.6 * m * h Joules I maintain it at that height and go to Earth. There, I will have: 9.8 * m * h Joules Where does the added energy come from ? I cannot see why it would come from the Moon Earth travel itself, as only the mass matters during the motion ... Thanks for your time and answers ! Cheers, Fabien.