Physics of Elastic Spheres Skipping on Water

It is well known that one can skip a stone across the water surface, but less well known that a ball can also be skipped on water. The Waboba R © is an elastic ball used in a game of aquatic keep away in which players pass the ball by skipping it along the water. We investigate the physics of skipping elastic balls to elucidate the mechanisms by which they bounce off water. High speed video reveals that, upon impact with the water, the ball creates a cavity and deforms significantly due to the hyperelasticity of the material; the flattened spheres resemble skipping stones. With an increased wetted surface area, a large hydrodynamic lift force is generated causing the ball to launch back into the air, onward to an awaiting Wabobian. Unlike stone skipping, the elasticity of the ball plays an important roll in determining the success of the skip. Through experimentation, we demonstrate that a deformation timescale during impact must be longer than the collision time in order to achieve a successful skip. Scaling for the deformation time is presented. Aided by a numerical model, we identify two dominate modes of sphere deformation. The effect of impact velocity and angle on the skipping behavior is also investigated. We hope that our findings lead to an optimized Waboba experience such that the painful alternative game of “skipping stone keep away” finally becomes obsolete.