geometry: the science of skipping stones

A cool, calm lake. Your dog frolicking along the beach. Your significant other admiring your attempt to skip a stone across the water — and, to your deep embarrassment, it plops straight down with nary a bounce. Has this ever happened to you? Thanks to a new study, you need never suffer the humiliation of inadequate stone-skipping again. Christophe Clanet from the University of Aix-Marseille in France and his colleagues report in the Jan. 1 Nature that they have discovered the secret of maximizing the number of bounces in a skipped stone: you need to keep an angle of about 20 degrees between the spinning stone and the water’s surface. To obtain that magic number, the researchers built a stone-skipping machine that fired aluminum discs into a tank of water. Using high-speed video cameras, Clanet and his team monitored the discs as they hit the water at various angles, velocities, and spins, focusing on the “crucial moment” of the bounce. Though the speed and rotation of the object are important, the physicists found that the angle of collision between the object and the water was critical to obtaining the largest number of bounces. In fact, no rebound was possible if the angle was greater than 45 degrees. So, the next time you want to impress your partner with your stone-skipping prowess, bring a protractor.

This news brief appeared in the Discoveries column of the Boston Globe’s Health/Science section on 1/06/2004.