[LB7 (OP)]

My device is massless just because I need to simplify the calculations. I can set a mass for all parts. But like that it is easier to find my error. If you say what you didn't understand I can explain. It is like gravity but I can change the orientation.  It is a theoretical device.

[Kryptid]

Ahem! A photon is massless. A photon has energy. You could argue it has relativistic mass.

I know that a photon has no rest mass (or invariant mass, if you prefer). Relativistic mass is indeed along the lines of what I was thinking.

[LB7 (OP)]

The device with the white disk (or another shape) fixed to the ground:

1/ the energy from the walls is constant
2/ the potential energy at start and at final are constant
3/ the potential energy to move out/in the layer 1/2 is constant (the sum of in + out = zero)
4/ I have THE CHOICE to move out any layer I want, but the pressure is NOT THE SAME so how the sum of energy could be constant ?

https://i.imgur.com/PnZEnq5.png

[LB7 (OP)]

I don't need to follow the principle of last action ! I have the choice. Only one energy is not fixed, the sum of energy is not constant.