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.

from the Boston
Globe