Skipping Stones

When physicist Lydéric Bocquet's 7-year-old son asked him why a good throw makes a stone skip on a lake instead [blank] sinking, 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 [blank] 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 surface. If the lift force is greater than the force of gravity then the stone bounces up; otherwise it sinks. Bocquet's physical model bears out the conventional wisdom that the best skipping stones [blank] flat and should be hurled nearly parallel to the water, fast and spinning. The stone's flatness maximizes the lift, as does its speed, [blank] also provides energy to keep it bouncing along. These are the same factors that keep water-skiers from sinking, Bocquet says. Spin prevents the stone [blank] tilting and hitting the water edge-first, just as fast rotations stabilize a bicycle or a spinning top.