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An incompressible liquid is never completely incompressible, more like quasi-incompressible. So when you apply considerable force $F$ on the piston, pressure will wise by say $\Delta p$:: $$\Delta p=\frac{F}{A},$$ where $A$ is the cross-section of the piston (and assuming constant $F$). But the liquid will have decreased slightly in volume by $\Delta V ... 1 The pressure at the bottom of the water tank is large. You have to push the balls in against this pressure - that is exactly as much work as you will get back. No free lunch in nature. Sorry. 1 What is the potential energy of the human body? 36 MJ$\simeq$8600 kcal. The basal metabolic state for a human is about 60 W. Let's assume (horrible) an immobile person dies after one week (604800 s) of starvation, he will have consumed 60 W * 604800 s = 36,288,000 J. Where does this energy come from? from the high energy chemical bonds of ... 1 Your principle ("you will loose half of the energy,... I feel that the principle is deeper and universal") does not work, and I will show that using a counterexample. Assume a similar example to the one you used with the water, but in this case the potential energy were$mgH^2$instead of$mgh$. In such a case$U=mgH^3/3\$, after splitting the water in ...