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I found this today on the interwebs:

Magic machine

Obviously this cannot work, but what bugs me is that I cannot figure out the part that would stop it and bring it to equilibrium. I suspect that it has to do something with the place where the balls enter the water - they would need to overcome some kind of a force that is larger than the buoyancy force of the balls already fully submerged. But I can't figure out what that is for a force and why it would exist there.

Can someone explain to me where this would fail?

Qmechanic
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Vilx-
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2 Answers2

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To insert one of the balls from the bottom, you have to provide $V\cdot \Delta P$ energy, where $V$ is the volume of a ball and $\Delta P$ is the difference between the pressure at the bottom of the tank and the atmosphere.

This energy, $V\cdot \Delta P$, is exactly the same energy that you get when the ball moves from the bottom to the top. Indeed, the pressure at the top of the tank is the same pressure of the atmosphere.

Doriano Brogioli
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When the ball enters the water at the lower end it needs to displace its own volume of water to get inside. This will raise the level of the water column which takes exactly the energy it gains later by floating up.

The rest is the usual friction.

done.

rauschi2000
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