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Heavier rocks produce a ‘super-elastic response’ when skimming.
Heavier rocks produce a ‘super-elastic response’ when skimming. Photograph: Michael Heffernan/Getty Images
Heavier rocks produce a ‘super-elastic response’ when skimming. Photograph: Michael Heffernan/Getty Images

Potato-shaped stones are better for skimming, say experts

This article is more than 1 year old

Scientists claim that heavier, rather than flatter, rocks can produce ‘almighty’ leaps out of the water

Scientists have identified particular types of stone that can produce “almighty” leaps out of the water when skimmed across the surface.

While aficionados of the pursuit favour thin, flat stones for long-distance skimming, the researchers’ mathematical model reveals that heavier, potato-shaped stones can achieve more dramatic results, which blast the rock into the air.

“Try some wacky stones and see what happens,” said Dr Ryan Palmer, an applied mathematician at the University of Bristol. “Try and throw a stone that looks like a potato. You can get some fun things happening with heavier stones.”

Palmer and his colleague Frank Smith, a professor of applied mathematics at University College London, created the mathematical model to investigate how the shape and mass of an object affect how it skims on the surface of water. Beyond the crucial implications for stone throwing, the model will help scientists working on more commercial problems, such as the buildup of ice on aircraft at altitude, and the forces at play when planes land on water.

Armed with the model, the researchers identified a mathematical relationship between the mass of the stone and the curvature of its underside, which determined whether or not it would skim. Heavier stones that would otherwise sink will skim if the curvature is sufficient, they conclude.

However, bulkier stones, even if thrown well, are not the sort to skip across the water a dozen or so times before disappearing beneath the surface. Heavier, more curved stones interact with the water in a different way, the scientists report in Proceedings of the Royal Society A.

“If you’ve got a heavier rock, you can get a super-elastic response, where you get a single mega-bounce rather than lots of little bounces,” said Palmer. “There’s this almighty leap out of the water.

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

According to the study, heavier stones that have a curved base can bounce off the water because the curvature changes how the rock contacts the water. On initial contact, it presses into the water more deeply and for longer. As a result, the pressure from the water is sustained over the bottom of the stone for longer, and the surface of the water deforms more, both of which act to push the stone upwards.

“You are basically turning a horizontal throw into more vertical motion. It leads to an increase in the force of the water pushing back on the rock and that can overcome the mass and drive the stone back out,” said Ryan. “If the stone was flat, it would be too heavy to skim.”

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

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