Stone Skipping Gets Scientific

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What surprised Bocquet and his colleagues, however, was that no matter how they changed speed and spin, stones always had the best chance at bouncing when they hit the water at an angle of 20 degrees.

Stones that enter the water at angles less than 20 degrees bounce but lose much of their energy dragging through the water, while stones that enter at angles greater than 45 degrees don't bounce at all—they sink.

Stone-Skipping Machine

Bocquet first got interested in the physics behind stone skipping after his eight-year-old son asked him about it while they, as millions of fathers and sons have done before, were skipping stones together at a river.

"I first thought about it as a funny hobby, a kind of wink where physics can help understand everyday life," he said. The hobby resulted in a set of equations published in the American Journal of Physics in 2002 describing the optimal speed and spin for a toss.

According to his equations, to match McGhee's Guinness Book of World Records pinnacle of 38 skips requires a stone tossed at 25 miles per hour (40 kilometers per hour) with a spin of 14 rotations per second.

The paper caught the attention of Christophe Clanet, a physics professor at the Institut de Recherche sur les Phènoménes Hors Equilibre in Marseille. As a way to get his students at Ecole Polytechnique in Paris interested in the scientific process, he had them construct the stone-skipping machine.

"The project went on really nicely," said Bocquet. "A real scientific apparatus was constructed controlling all the parameters of the throw and recording it using a high-speed camera, which is a really difficult and tricky task."

Using the machine, which launches aluminum discs across a pool of water, the researchers arrived at the "magic angle" of 20 degrees.

In the near future, Bocquet said he and his colleagues hope to attempt the world record with their machine, testing the equations and theory of what's required to achieve the maximum number of bounces.

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