Reading and writing fill in the blanks
When children innocently ask head-scratching questions such as "Why is the sky blue?", most parents manage only some sheepish hems and haws. But when physicist Lydéric Bocquet's 7-year-old son asked him why a good throw makes a stone skip on a lake instead of , Bocquet took the question as a challenge. He has now produced a set of equations explaining the physics underlying the popular pastime.
Two key forces act on a skipping stone: gravity, which pulls it down, and lift, the reactive force of the water, which pushes the stone up each time it hits the . If the lift force is greater than the force of gravity then the stone bounces up; otherwise it sinks. Bocquet's physical model out the conventional wisdom that the best skipping stones are flat and should be hurled nearly parallel to the water, fast and spinning. The stone's flatness maximizes the lift, as does its speed, which also provides energy to keep it bouncing along. These are the same factors that keep a water-skier from sinking, Bocquet says. Spin prevents the stone from tilting and hitting the water edge-first, just as fast rotations a bicycle or a spinning top.