News summary produced by Claude AI
A team of mathematicians led by Leif Ristroph at New York University’s Courant Institute has experimentally resolved Feynman’s Sprinkler Problem, a long-standing question in physics concerning how sprinklers operate when reversed. The research, published in the Proceedings of the National Academy of Sciences, demonstrates that the angular momentum of water flows drives sprinkler rotation across multiple device designs.
The sprinkler problem has intrigued physicists for decades, particularly since physicist Richard Feynman documented his own unsuccessful attempts to investigate it experimentally during the 1980s. Previous work conducted in 2024 established that a reverse sprinkler rotates approximately 50 times more slowly than a conventional sprinkler, despite both relying on related physical mechanisms. A standard sprinkler operates similarly to a rotating rocket, with water ejected from arms generating forces that cause rotation. In reverse mode, water flows inward toward a central chamber where incoming jets collide, and their slight misalignment produces forces causing rotation in the opposite direction.
The current investigation expanded upon the 2024 findings by testing silly sprinklers with curved and looping tubes, examining whether more complex shapes might behave differently from standard S-shaped designs. The research team constructed multiple devices and tested each in both forward and reverse configurations, measuring rotational speed, internal and external water movement, and torque when rotation was prevented. These experiments allowed researchers to evaluate competing explanatory theories, including Ernst Mach’s 19th-century hypothesis and Feynman-associated theories. The results consistently supported the momentum flux theory, which emphasizes how swirling water carries momentum through the sprinkler, while ruling out alternative explanations based on fluid rotation direction or water movement near arm edges.
Beyond settling the scientific puzzle, the findings have practical implications for engineering applications. Understanding how objects respond to moving fluids could improve the design of turbines and other devices that convert energy from flowing liquids. The research demonstrated that sprinkler arm shapes can be modified to alter water jet behavior, potentially offering useful design capabilities for fluid-based technologies. The work was supported by the National Science Foundation and involved contributions from graduate students and undergraduate researchers at New York University, as well as a co-author from Colorado School of Mines.