Skipping stones 101

Bouncing rocks across the local pond may be the epitome of casual amusement, but that’s no reason not to use a bit of physics to improve your technique.

To achieve controllable, repeatable tosses in the laboratory, Christophe Clanet of the Institut de Recherché sur les Phénomènes Hors Équilibre in Marseille, France, and his colleagues devised an electric-powered catapult that hurled spinning, cookie-size model stones–actually aluminum disks–into a vat of water as long as a bathtub. The experiment focused on the stones’ initial impact with the water.

The team could dial in the spin rate, speed, and angle of approach for each toss. By means of high-speed video, the scientists evaluated some 200 disk-water collisions, measuring details such as the shape of the temporary dent made in the water and the length of time that each contact lasted.

In the Jan. 1 Nature, Clanet and his coworkers report that a 20 upward tilt of the disk with the trailing edge making the contact saps the least energy from the moving disk and therefore should lead to the greatest number of skips. To test that prediction, the researchers plan to bring their catapult and a supply of disks to a 30-meter-long pool, Clanet says.


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