This past July I ventured out of my air-conditioned Minnesota home into the superheated and smoky outdoors only to move my sprinkler around the lawn. Coincidentally, I learned about an ingenious experiment by revealing that Centrifugal Flows Drive Reverse Rotation of Feynman’s Sprinkler.* The twist on this story, as illustrated by the video, is that the sprinkler is submerged with water being sucked into its arms.

Given all the misery this summer throughout North America and Europe, it is refreshing to spend time on a physics puzzle that is finally solved. It’s good to know that my sprinkler will rotate opposite at about one-fortieth the normal rate if set upside down in our grandkiddy pool with the water flow reversed (that part being a big challenge).

*Updated on July 13 this year with research on “silly shapes,” which revealed how Geometry controls momentum flux in the sprinkler problem.

P.S. The leader of the team who solved this mystery is Doctor Leif Ristroph, an NYU physics professor. Believe it or not, one of his team for the research reported in original article was Doctor Brennan Sprinkle, now a professor of applied mathematics and statistics. Sprinkle is now working on a computer simulation to provide more insights on the Feynman problem. It was meant to be!