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The Sciences

Scientists Perfect the Skipping Stone

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An elastic sphere bounces off the surface of the water, captured by a high-speed camera. (Credit: Chris Mabey/Randy Hurd/Splash Lab) A friendly trip to the beach often sparks a casual competition to see who is more skilled in the art of skipping a stone. But before the first attempt, a tactful stone-skipper will examine the inventory of seaside rocks to find a one uniquely shaped for the task. Scientists at the aptly named Splash Lab at Utah State University have perfected the skipping stone. Through a series of experiments that applied scientific rigor to our favorite lazy beach activity, they determined that a squishy sphere will maximize the number of skips. Along with the Naval Undersea Warfare Center, Splash Lab head Tadd Truscott and his team experimented with different shapes and materials to create a squishy sphere that maximizes the number of skips from a single throw. They published their findings Friday in the journal Nature Communications. “Our approach was playful at first,” said Truscott in a press release. “My son and nephew wanted to see the impact of the elastic spheres in slow motion, so that was also part of the initial motivation. We simply wondered why these toys skip so well. In general, I have always found that childish curiosity often leads to profound discovery.” [embed]https://www.youtube.com/watch?v=1jyyqvrs_oo[/embed]

The Water Pushes Back

When you skip a flat stone across the water, the impact creates a shallow cavity in the water. If the angle is flat, and the stone is wide enough, surface tension and inertia combine to catapult the stone back up into the air and onward on its journey. As most of us know, thin flat stones work best, as these maximize surface area while keeping weight low — ideal for bouncing off the water. Truscott discovered that the rigidity of skipping stones was actually holding them back. Using high speed cameras, he watched how rigid and supple spheres behaved when skipped across the water. He found that more pliant objects stayed closer to the surface of the water, allowing them to conserve more momentum. His team could even predict how many times an object would skip based on their models.

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A comparison of how rigid spheres (top) bounce across the water compared with elastic spheres (bottom) (Credit: Chris Mabey/Randy Hurd/Splash Lab)

Perfect Shape Conforms to Water

By creating an elastic sphere out of silicone rubber, his team improved on nature’s design. Their sphere contorted and stretched into the perfect skipping shape no matter the angle it was thrown from. Truscott says that his research has applications that extend beyond a trip to the beach. Insights into how flexible materials react when impacting the water could help improve the design of boat hulls and life jackets, for example. We can only imagine what Kurt Steiner, who owns the Guinness World Record for skipping a stone 88 times, would do with one of Truscott's specialized spheres.

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