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I've googled for about 5 minutes trying to understand ....is the "reverse sprinkler" just a device that has been impossible to build so it was a strictly theoretical problem? "We found that the reverse sprinkler spins in the ‘reverse’ or opposite direction when taking in water" - really, this was that impossible to build that nobody knew what it would do? weird

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From stories I’ve read, I think people generally, and including Feynman, concluded that the sprinkler would not rotate. Because conservation of angular momentum. Which is almost true. So what’s fun is that this experiment shows it’s not entirely true, if you have a sensitive enough experiment that takes into account more nuance of the fluid dynamics.

It's a sprinkler with the pumps pulling water in instead of pushing it out.

Plenty of people built one since Feynman stated the problem, it simply doesn't rotate or rotates very weekly on the same direction a normal sprinkler would. A lot of people think this is anti-intuitive.

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