The Feynman Reverse Sprinkler Problem Solved, Also Applicable to "Silly Sprinklers"

Based on an article reported by Jennifer Ouellette for Ars Technica.

Watering your lawn in the summer can be both pragmatic and fun with so-called “silly sprinklers,” designed to create amusing loops and spirals of water jets. And there’s some fascinating physics at work to boot. Researchers at New York University’s Courant Institute conducted a series of experiments with different silly sprinkler designs to find the answer to a longstanding problem in fluid dynamics, according to a new paper published in the Proceedings of the National Academy of Sciences.

As previously reported, the reverse sprinkler problem is associated with physicist Richard Feynman because he popularized the concept, but it actually dates back to a chapter in Ernst Mach’s 1883 textbook The Science of Mechanics (Die Mechanik in Ihrer Entwicklung Historisch-Kritisch Dargerstellt). Mach’s thought experiment languished in relative obscurity until a group of Princeton University physicists began debating the issue in the 1940s. Feynman was a graduate student there at the time and threw himself into the debate with gusto, even devising an experiment in the cyclotron laboratory to test his hypothesis.

The reverse sprinkler problem questions whether a sprinkler that sucks in water in the reverse direction will rotate. Intuitively, one might expect the sprinkler to rotate in the opposite direction of its usual motion. However, the actual physics is more complex, and past experiments have reported varying results. The new research confirms the “momentum flux theory,” published in 2024, which clarifies how the angular momentum of water drives rotation. Furthermore, the study demonstrates that the same theory can be applied to the operation of “silly sprinklers,” which spray water in loops and spirals.

Editorial Opinion

In the short term, over the next three to six months, the findings of this research could influence the field of fluid dynamics education. Drawing a comparison between reverse sprinklers and silly sprinklers has the potential to become a new standard topic in undergraduate physics experiments. In addition, sprinkler design manufacturers might use this theory to optimize nozzle shapes and water flow rates. However, at present, this remains foundational research and may not immediately lead to product improvements.

In the long term, over the next one to three years, the study is expected to have broader implications for the efficient design of fluid devices in general. Applications could include water turbines, pumps, and even microfluidic devices. Particularly in industrial processes requiring the control of turbulence and vortices, momentum flux theory may provide a new design framework. While further engineering verification is necessary for practical applications, this foundational physics discovery lays the groundwork for future innovations.

From an editorial perspective, one key question is how much of conventional wisdom might be overturned by the resolution of this classical physics puzzle through modern measurement techniques. Another intriguing aspect is how a common toy like the silly sprinkler has served as a platform for examining unresolved scientific questions, which highlights the fascinating intersection of everyday objects and advanced science communication.

References

• “Solution to Feynman’s reverse sprinkler puzzle also applies to “silly sprinklers"", by Jennifer Ouellette — Ars Technica, 2026-07-13T19:00:47.000Z (CC BY-NC-ND)
• Source URL: https://arstechnica.com/science/2026/07/solution-to-feynmans-reverse-sprinkler-puzzle-also-applies-to-silly-sprinklers/