Physicists are drawn to complex puzzles encompassing the nature of space, time, and the origins of the universe. Surprisingly, even the rotation of lawn sprinklers captures their curiosity.
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Physicists
A recent experiment tackles an intriguing physics problem introduced by physicist Richard Feynman in the 1980s. The conundrum revolves around a specific sprinkler design that propels water from the ends of an S-shaped tube, causing the sprinkler to spin in the opposite direction due to the conservation of angular momentum. This aspect is well-understood.
However, the puzzle arises when the same sprinkler is submerged in a water tank and operates by drawing water in. The question seems deceptively simple, yet disagreements among physicists emerge, suggesting that the sprinkler should either spin in the opposite direction or remain stationary. Previous experiments yielded conflicting results.
Applied mathematician Leif Ristroph and his team undertook the challenge, conducting experiments with a meticulously crafted transparent sprinkler and supporting their measurements with mathematical calculations. The device, floating in a water tank to minimize friction, revealed that when operated in reverse, the sprinkler indeed spins in the opposite direction, solving the long-debated physics puzzle, as reported in a paper accepted to Physical Review Letters. Further investigation unveiled the reason behind this phenomenon.
In contrast to the normal sprinkler, which expels water outward, the reverse operation generates jets of water within the sprinkler itself. These jets create vortices, observed through laser illumination of microparticles in the water. Notably, these jets and vortices lack symmetry. When the jets collide within the sprinkler’s central region, they continue at an angle, indicating a glancing collision rather than a head-on impact.
The asymmetry in the internal jets, even though the sprinkler arms are perfectly aligned, disrupts the water flow in each jet as it travels through the curved arms. This asymmetry, crucial for conserving angular momentum, causes the sprinkler to rotate backward when releasing the jets, providing a resolution to the longstanding physics puzzle.
