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First, remove the flaps and replace them a single flap (or valve) where the balls enter the water on the bottom right which prevents the water to flow out. In order to get the red ball into the water, you'll then have to overcome the excess pressure corresponding to the water column, which will cost an energy exactly equal to the energy gained by the balls ...


The 'starting state' for the gas in a convergent-divergent nozzle is not just at high temperature, but also at a high pressure. The 'final state' for the exhaust gases is at a lower temperature, but also at a lower pressure (ideally the exhaust pressure is equal to the ambient pressure for real-life rockets). If you wanted to construct a perpetual motion ...

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