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Hitting the water at a magic angle gives top performance in a time-honoured pastime.
Skipping stones across water has been a popular pastime for thousands of years - the rules of the game have remained unchanged since the time of the ancient Greeks' - and the world record, set by J. Coleman-McGhee in 1992, is believed to be 38 rebounds2. Following earlier attempts3-6 to analyse the physics of this ancestral human activity, we focus here on the crucial moment in stone skipping: when the stone bounces on the water's surface. By monitoring the collision of a spinning disc with water, we have discovered that an angle of about 20° between the stone and the water's surface is optimal with respect to the throwing conditions and yields the maximum possible number of bounces.
A stone-skipping throw involves four parameters (Fig. 1): U and Ω are the translational and spin velocities, respectively, a is the 'attack' angle of the stone in relation to the water's surface, and β is the impact angle of the translational velocity. Our experimental set-up is designed to control for each of these parameters independently. Collision sequences were recorded using a high-speed video camera (Fig. 1a), allowing factors such as collision time, change in orientation of the stone, and the shape of the liquid cavity to be determined.
Regarding the role of spin velocity, rotation is found to stabilize the stone (as...