Feynman’s Sprinkler Solution Decodes “Silly Sprinklers” Too
▼ Summary
– Silly sprinklers form the basis of experiments by New York University researchers to solve the reverse sprinkler problem in fluid dynamics.
– The reverse sprinkler problem was popularized by Richard Feynman but originates from Ernst Mach’s 1883 textbook.
– Feynman, as a graduate student at Princeton, conducted an experiment in a cyclotron lab to test his hypothesis on the reverse sprinkler’s motion.
– Mach proposed that a reverse sprinkler would not rotate because the forces from sucking in water cancel each other out.
– Feynman’s experiment showed only a slight tremor when pressure was applied, with the sprinkler then returning to a still position.
When summer arrives and you reach for a silly sprinkler to cool off the lawn, you might notice those playful loops and spirals of water. But behind that whimsical spray lies a puzzle that has stumped physicists for over a century. Researchers at New York University’s Courant Institute have now cracked the code. Their experiments, detailed in a new paper in the Proceedings of the National Academy of Sciences, used different silly sprinkler designs to solve a classic riddle in fluid dynamics.
The so-called reverse sprinkler problem is often credited to physicist Richard Feynman, who made it famous. But its roots go deeper, tracing back to a chapter in Ernst Mach’s 1883 textbook The Science of Mechanics. That thought experiment stayed in the shadows until the 1940s, when a group of Princeton University physicists revived the debate. Feynman, then a graduate student, jumped in with characteristic enthusiasm. He even built an experiment in the cyclotron lab to test his ideas.
At first glance, you might assume a reverse sprinkler would simply work in reverse, spinning the opposite way. The reality is far more nuanced. As Feynman wrote in Surely You’re Joking, Mr. Feynman (1985), “The answer is perfectly clear at first sight. The trouble was, some guy would think it was perfectly clear [that the rotation would be] one way, and another guy would think it was perfectly clear the other way.”
Mach originally predicted no rotation at all. His reasoning: the reaction force on the nozzle as it sucks in water pulls it one way, while the water flowing into the nozzle pushes it the other way. In a steady state, these two forces cancel out. Feynman’s own experiment confirmed a slight tremor when pressure was first applied, but then the sprinkler returned to its starting position and stayed still. Now, the NYU team has shown exactly how and why that happens, finally putting a definitive answer to the Feynman sprinkler puzzle.
(Source: Ars Technica)