Feynman sprinkler

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A Feynman sprinkler, Feynman inverse sprinkler or most correctly reverse sprinkler began as a thought experiment in explaining the workings of the reverse of a typical rotary lawn sprinkler. This experiment is commonly associated with and named after Richard Feynman though he did not come up with the problem or ever publish a solution to it, only help popularize it in Surely You're Joking, Mr. Feynman! and other writings of his.^[1]

[edit] Explanation

The problem concerns what would happen if such a sprinkler were submerged in water and water were sucked through it, flowing backwards. The possible outcomes are as follows: the sprinkler will spin forward; the sprinkler will spin backward; the sprinkler will not spin at all. There has been much debate in the past century over the solution^[2]. In recent years, the experiment has been performed many times, (usually with air, not water, as the medium of the sprinkler), demonstrating that the "reverse" sprinkler does not spin in the steady state.

However the University of Maryland did the experiment on a very low friction bearing, and found that the sprinkler did indeed turn very slowly as if being sucked forwards, and so the debate continues.^[3]