Experiments provide new insights into the reverse sprinkler problem

For decades, physicists have puzzled over Richard Feynman's famous "reverse sprinkler" problem: a lawn sprinkler that blows water out will rotate, but if you reverse it to suck water in, does it spin the opposite way? Physics World reports that researchers at New York University have now solved it. They built custom submerged sprinklers with different arm geometries—spiral shapes, hookback designs—and systematically tested which physical principle explained the results. They found it wasn't the overall angular momentum of the system, but rather subtle asymmetries at the center where the arms begin, which inject angular momentum as fluid flows inward. That's why the reverse sprinkler rotates much more slowly. While there's no revolutionary lawn sprinkler upgrade coming, the elegant experimental design is helping physicists develop better computer simulations of open fluid systems—a rigorous testing ground for methods that could apply far beyond this classic puzzle.

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