Updated

Updated · Tech Times · Jul 19

NYU Team Solves 143-Year Feynman Sprinkler Problem, Pinning Rotation on Momentum Flux

Updated

Updated · Tech Times · Jul 19

NYU Team Solves 143-Year Feynman Sprinkler Problem, Pinning Rotation on Momentum Flux

Q: What simple insight solved the physics problem that stumped even Richard Feynman?

The key insight was that sprinkler motion is set by **momentum flux**—the angular momentum carried by the water jets—not by swirl outside the device or by a simple reversal of the forward-flow case. In the normal sprinkler, water shoots outward and acts like a rocket, producing strong torque. In the reverse sprinkler, the crucial discovery was that the incoming streams **converge inside the hub and do not collide perfectly head-on**. That slight off-axis collision creates a small net torque in the **opposite direction**. That explains both parts of the old puzzle: the reverse sprinkler really does turn backward, and it does so far more slowly—about **50 times slower**—because the inward-flow torque is much weaker than the outward jet thrust. The experiments also showed why the problem misled generations of physicists, including Richard Feynman. Fluid flow is not simply reversible in real life: sucking water in from all directions is physically different from blowing a concentrated jet out. The team’s tests with custom “silly sprinklers” showed that **arm geometry** controls how the jets carry and exchange angular momentum, settling the issue and ruling out earlier theories associated with Ernst Mach and Feynman.

3 articles · Updated · Tech Times · Jul 19

A NYU Courant-led team reported the first comprehensive experimental resolution of the 143-year-old Feynman sprinkler problem, finding angular momentum carried by fluid jets governs rotation in both normal and reverse flow.
Using custom "silly sprinklers" with multiple arm geometries, the researchers measured motion and torque in both modes and showed arm shape controls how jets form and transfer torque.
The PNAS study ruled out two long-debated explanations—Mach's swirl theory and Feynman's outer-flow theory—because neither matched rotations and torques across the tested designs.
The experiments also explain why reverse sprinklers spin about 50 times slower: inward jets collide off-axis inside the hub, producing a much weaker torque than outward rocket-like jets.
Beyond settling a puzzle that stumped researchers for decades, the result could help engineers optimize bidirectional-flow devices such as reversible turbines and tidal energy systems.

Sources

Tech Times2d ago

NYU Courant Institute Team Solves Feynman Sprinkler Problem, Citing Momentum Flux

ScienceDaily1d ago

NYU Scientists Solve Feynman's Sprinkler Problem Using "Silly Sprinklers"

Facebook2d ago

Each summer, lawns are marked by a familiar addition: “silly sprinklers,” whose loops and spirals spew water in creative ways. While seemingly frivolous in their construction, a team of mathematicians has used

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The Feynman Sprinkler Problem Settled After 143 Years: NYU’s Landmark 2026 Findings

Overview

After 143 years of debate, the Feynman Sprinkler Problem—first posed by Ernst Mach in 1883 and later popularized by Richard Feynman—has finally been solved by Leif Ristroph's team at New York University. Their groundbreaking experiments, published in July 2026, provided the definitive answer to whether a reverse sprinkler rotates and in which direction. This research ended decades of scientific disagreement and put to rest a puzzle that had challenged physicists since its inception, marking a major milestone in fluid dynamics.

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