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Letting the frogs out, baking pancakes, and making rabbit leaps – whatever you call it, the universal and simple game of skipping stones is science made tangible and fun.
Fifty-one skips! When I heard it, the stone-skipping world record boggled my mind a bit. Having never achieved more than two or three skips myself, I almost didn’t believe it was possible, until I was treated to a demonstration of multiple 10- and 15-skip throws by a friend. (His increased skill probably has to do with having studied physics, and/or having few childhood hobbies.)
As evidenced by the playful names given to the game (translations from different languages, above), stone skipping is enjoyed the world-over. And wherever, launched by human hands, stone attacks water, a down-to-earth demonstration of physics is taking place. Skipping stones can even be an interplanetary pastime, in a way: finding flat stones on Mars is helping scientists make inferences about the past presence of water on the Red Planet. (The piece behind the link, by Mars Science Laboratory scientist John Grotzinger, is a great example of relatable science writing that connects the astronomical to the everyday.)
So what’s the secret to stone skipping? As with other precision games, it’s all in the wrist. Using what amounted to a Frisbee-throwing machine that repeatedly hurled discs into water, a group of French researchers determined that 20 degrees to the water surface is the best entry angle for skipping. The spin of the stone is crucial for stabilization while the water provides the optimally angled stone an upwards force that launches it back out of the water, provided the angle is shallow enough.
An earlier paper by one of the same authors found that the then-world record of 38 skips required a 43 km/h throw with 14 spins per second. The starting speed is of course important, influencing how many bounces the stone can make, but it’s the initial spin that is the limiting factor, as evidenced by the destabilization of the stone and the increase of tiny bounces (or “pitty-pat”) just before it sinks. A light, flat stone is generally also a prerequisite.
Stone skipping offers a remarkable number of lessons: it’s the simple game that first sparks a child’s interest in science; it’s the ability of humans to manipulate the world through brute force and ingenuity; it’s the game of skill with a repetitive nature that our competition-hungry species is so fond of; it’s the phenomenon that appears magical but ultimately boils down to an interaction of forces that is not supernatural. Skipping stones is a scale model of more complex maneuvers that also require a meticulous understanding of the underlying forces, a bizarre spectacle whose causal dynamics can be harnessed to do something even crazier, like landing a robot remotely on another planet with a sky crane. Who knows what simple diversion could inspire future scientific breakthroughs.
More on the physics of skipping stones: Discover Magazine