Experts say that stones in the shape of potatoes are better for skimming

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Scientists have identified certain types of stone that can produce “great” leaps of water when it is skimmed across the surface.

While stalking enthusiasts prefer thin, flat stones for long-distance skimming, the researchers’ mathematical model reveals that heavier, potato-shaped stones, which blast the boulders in mid-air, can achieve more dramatic results.

“Try some exotic stones and see what happens,” said Dr Ryan Palmer, an applied mathematician at the University of Bristol. Try throwing a stone that looks like a potato. You can get some fun stuff with the heavy stones.”

Palmer and his colleague Frank Smith, Professor of Applied Mathematics at University College London, created the mathematical model to investigate how the shape and mass of an object affects how it peels off at the surface of the water. Besides the critical effects of stone throwing, the model will help scientists work out more commercial problems, such as ice buildup on planes at altitude, and the forces that come into play when planes land on water.

Armed with the model, the researchers determined a mathematical relationship between the stone’s mass and the curvature of its underside, which determines whether or not it will energize. They concluded that heavier stones that would otherwise sink will skim if the curvature is sufficient.

However, larger stones, even if well thrown, are not of the kind to be skipped through the water ten times or so before disappearing below the surface. The scientists said in the Proceedings of the Royal Society A.

“If you have a heavier rock, you can get super-elastic response, where you get one huge bounce instead of a lot of little bounces,” Palmer said. “There’s this huge jump out of the water.

“It changes the game. It’s very satisfying if you can make it happen.”

According to the study, heavy stones with a curved base can bounce off the water because the bending changes how the rocks come into contact with the water. Upon initial contact, it presses into the water more deeply and for a longer duration. As a result, the water pressure above the bottom of the stone lasts longer, and the surface of the water deforms more, both of which serve to push the stone up.

“You’re basically converting the horizontal throw into a more vertical motion. It creates more force pushing the water back on the rock and it can overcome the block and push the stone out,” Ryan said. “If the stone were flat, it would be too heavy to cut.”

The model did not look at the effect of spinning the stones, but previous research has shown that spinning is an important part of successful stone skimming. The main effect is to stabilize the stone through gyroscopic forces as it slides through the air. If the stone was thrown well in the first place, the spin can prevent it from spinning mid-flight and hitting the water at a bad angle.

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