The drawing show a barometer with water inside. If I move up object with air inside I recover PV energy (P=external pressure, V=volume of object). If I want to move out object when it is at top, I need PV, is that ? So if it's that. If I replace: 1/ gravity by balls attracted with springs (for have law of gravity in blue column) 2/ atoms with empty balls with very thin walls 3/ pressure with velocity of empty balls 4/ object to move up with some empty balls I can disappear empty balls at top: at top we can recover energy from velocity and let at top only the small walls of balls. If walls are very very thin, I let virtually nothing. With friction = 0 where the energy is lost ?