What’s the record for the number of times a rock hurled by a person has skipped across a body of water? The answer: a whopping 88 skips in a row. The feat was accomplished using a rock shaped like a smooth disk—long thought to be the best type of skipping object. But it turns out that flat stones might actually not be the ultimate skippers.
A group of scientists from Utah State University, lead by physicist Tadd Truscott, recently tested differently shaped objects made of multiple materials to see which had the greatest skipping abilities. They found that a round ball made of elastic (flexible) silicone actually skipped better than a flat, hard stone.
A toy called a Waboba ball, which can bounce over water with much more ease than a stone, inspired Truscott to conduct the skipping study. His son and his nephew had expressed interested in seeing in slow motion what happened when a Waboba whizzes across water. Truscott had the perfect place to test the toys—his Splash Lab, which studies the behavior of fluids (substances with no fixed shape, like liquids or gases).
In the lab, Truscott and his team created their own versions of the toys out of solid balls of silicon wrapped in lycra, the slick material used for wetsuits. Rather than throwing the spheres themselves, the scientists built a launcher that shot the balls into tanks of water at speeds up to 160 kilometers (100 miles) an hour. Doing this ensured that the spheres hit the water’s surface at just the right angle. The team captured how the balls skipped with a high-speed camera.
When an object skips, it creates a shallow dip in the water. The depression increases the water’s surface tension. This force, which holds molecules of a liquid together, causes the surface of the water to spring back up like a trampoline, catapulting the object back into the air.
Truscott found that when skipped, a flexible ball would flatten out into the perfect disk-like skipping shape. The elastic object also stayed closer to the water’s surface than a rigid stone. This allowed the squishy sphere to conserve its momentum (a moving object’s mass times its velocity) so it could skip along for a greater amount of time. Rocks, on the other hand, dipped down into the water farther, causing them to lose momentum. As a result, they didn’t end up bouncing as far.
How elastic objects react with the surface of water doesn’t apply just to skipping stones, says Truscott. He thinks his study could be used to design boats with rubber hulls that might skim across water more efficiently than metal or wooden ones.