1. The problem statement, all variables and given/known data It is a question I asked to myself because I need to reply to another one. I imagined this case: http://imageshack.com/a/img912/7730/R964Vq.png [Broken] The step2 needs the energy E1, the step3 needs nothing because the gravity is 0 (I imagine the device between the Earth and the Moon), the air container is fixed, at step4 I recover E1, at step5 I recover an energy E2. At final I have -E1+E1+E2 = E2. 2. Relevant equations -- 3. The attempt at a solution With a solid I'm agree to put the force of weight in the center of mass and if I do that the energy is conserved. But why I need to do that with a fluid ? The force from the weight is at the bottom of the container not in the center of mass. If I use the true force in the bottom of the container, I need to add an upward force ?