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A sprinkler that sucks in water instead of spraying it sparked one of physics' longest debates
A Feynman sprinkler is a sprinkler-like device submerged in a tank that sucks the surrounding fluid in rather than pushing it out. A normal sprinkler spins because water jets push against nozzles set at angles, and the reaction turns the wheel. The reverse version seems like it should spin the opposite way, but the question of which way it turns became an intense and remarkably long-lived debate. In practice it usually stays still, because the inflow pulls fluid symmetrically from all directions. With very low friction and a fast inflow, it has been observed to turn weakly in the reverse direction. The puzzle is named after Richard Feynman, who mentions it in his memoirs, though the problem predates him and he never published a solution.
A conventional sprinkler spins because the jets exit at angles, and the reaction force on the nozzles creates torque. In the reverse case, fluid enters the nozzles from all directions, so the forces are balanced and the device has little reason to turn. Only with minimal friction does a small residual effect appear, and it can be surprisingly hard to predict from simple reasoning.
It is a clean example of how intuition about forces can fail, because pulling and pushing do not always produce mirror-image effects. It shows how a simple thought experiment can keep experts arguing for decades.
Richard Feynman's bestselling memoir Surely You're Joking, Mr. Feynman! is where he discusses the problem.
From Wikipedia, the free encyclopedia · text CC BY-SA 4.0
Illustrative photo · Cmglee, CC BY-SA 4.0, via Wikimedia Commons
