I recently saw a video on Youtube about a perpetual motion machine. Here is the link:


It's on Robert Boyle's flask. Where does the energy come from?

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    $\begingroup$ Short answer: No. Look up the 2nd law of thermodynamics. $\endgroup$
    – vnb
    Commented May 29, 2014 at 16:52
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    $\begingroup$ Nice video :-) It isn't perpetual motion because it's driven by the formation of carbon dioxide bubbles in the liquid. The foam in the tube to the right has a (much) lower density than the liquid in the flask to the left so the liquid moves down and pushes the foam up. The flow will stop when all the CO$_2$ has come out of solution. $\endgroup$ Commented May 29, 2014 at 16:57
  • $\begingroup$ Betteridge's law applies once again. $\endgroup$ Commented May 29, 2014 at 16:59
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    $\begingroup$ Why are people downvoting this? It's a perfectly reasonable question. To a non-physicist (and possibly to many physicists!) the video does look like a perpetual motion device, though of course it has a perfectly good explanation that Alan has supplied. Actually I think it's a witty experiment and one that would be great for tantalising students :-) $\endgroup$ Commented May 29, 2014 at 18:03
  • $\begingroup$ @JohnRennie I think it's quite fair to downvote any question asking whether perp motion exists. It's on the edge of acceptable to say "how does XYZ machine keep going?" but even that should be in jamesrandi.stackexchange.com :-) $\endgroup$ Commented May 29, 2014 at 18:58

1 Answer 1


I was surprised to see an effect that's actually real, and not a hidden motor or something like that. I believe this experiment can feasibly be repeated. The principles behind it make sense.

The driving force comes from the density difference in the cup versus in the pipes. The cup has very few bubbles in it compared to the pipe. Why? Because:

  1. The bubbles in the cup have an exit strategy - floating to the top. Not so in the pipe.
  2. The pipe has greater wall area to volume ratio. So it likely has more nucleation sites for the bubbles.

If I were recreating this, I would design the pipe to turn horizontal/vertical as close as possible to the bottom of the cup, in order to help maximize the driving pressure. Also, make sure to use the soda/beer right after opening it.

This experiment shows (not perpetual motion) that carbonation contains stored energy in some sense. More specifically, the process of a carbonated drink decaying into un-carbonated liquid and CO2 gas liberates extra energy. A small fraction of that energy is harvested here to drive the flow.

Very good science project.

In order to demonstrate that it is not perpetual motion, either allow it to run to its full conclusion, or try it again with soda that has sat out for a day. The data should support the hypothesis that the driving force to power the flow comes from stored energy in the carbonation.


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