NYU Researchers Experimentally Resolve the Century-Old Feynman Sprinkler Problem

Resolving a Century-Old Physics Mystery

For decades, the behavior of a reverse sprinkler—a device that sucks fluid in rather than spraying it out—has vexed physicists. While a standard sprinkler’s rotation is intuitive, driven by the reaction force of water exiting its nozzles, the reverse scenario has long been a subject of debate. The problem became famously associated with physicist Richard Feynman, who explored it during his time as a graduate student at Princeton University in the 1940s. Feynman even conducted experiments in a cyclotron laboratory to test his hypothesis, though he often discouraged naming the problem after himself, noting that it built upon ideas first explored by physicist Ernst Mach in his 1883 textbook, The Science of Mechanics (Die Mechanik in Ihrer Entwicklung Historisch-Kritisch Dargerstellt).

Mach theorized that there would be no rotation with a reverse sprinkler, arguing that the reaction force on the nozzle as it sucks in water pulls the nozzle counter-clockwise, while the water flowing into the inside of the nozzle pushes it clockwise. In this steady-state scenario, Mach argued the two forces would cancel each other out. As Feynman wrote in his 1985 memoir, Surely You’re Joking, Mr. Feynman!, 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. Feynman’s own experiment showed a slight tremor when pressure was first applied, and then the sprinkler returned to its original position and remained still.

Experimental Breakthroughs at the Courant Institute

The research team at New York University’s Courant Institute School of Mathematics, Computing, and Data Science moved beyond theoretical debates by constructing specialized apparatuses to measure the precise forces involved. By testing various shapes of silly sprinklers—the playful, colorful lawn devices found in backyards—the team identified how fluid inertia and pressure gradients dictate movement. Unlike previous efforts, the NYU researchers successfully induced rotation in their reverse sprinklers. Previous investigations had indicated that a reverse sprinkler spins approximately 50 times slower than a conventional one.

This work provides the experimental answer for Feynman’s Sprinkler Problem by showing, across several sprinkler types, how the angular momentum of water flows drives sprinklers’ rotation, said senior author Leif Ristroph, an associate professor at New York University’s Courant Institute School of Mathematics, Computing, and Data Science. The findings indicate that the rotation is governed by the momentum flux theory, which explains how swirling water carries momentum through the sprinkler system. This confirms that the direction and speed of the sprinkler are predictable consequences of the water’s interaction with the device’s geometry. The study represents a continuation of the team’s work, building upon initial findings reported in an earlier 2024 paper.

Engineering Applications and Future Fluid Dynamics

Beyond settling a historical scientific debate, the research offers tangible value for modern engineering. Understanding how structures respond to fluid flows is critical for the development of devices that convert liquid energy into power. As noted by co-author Brennan Sprinkle, an assistant professor at Colorado School of Mines, Our findings provide a firmer understanding of how components respond to fluid flows—knowledge that can guide future engineering and technological advances for devices, such as turbines, that convert these flows into energy.

The project, which received support from the National Science Foundation, involved a collaborative effort between faculty, graduate students, and undergraduates. By successfully applying empirical methods to a question that once relied on contradictory thought experiments, the team has provided a definitive resolution to a problem that has challenged physicists since the mid-20th century.

Sources: newsy-today.com, news.ssbcrack.com.