The Science of Skipping Stones

A flat stone thrown at the right angle across still water can bounce many times before finally sinking, and the physics behind it comes down to a careful balance of speed, angle, and spin. As the stone strikes the water, it pushes down on the surface, and the water pushes back up with roughly equal force. If the stone hits the water at too steep an angle, most of that force simply slows the stone down and it sinks. But if the stone is thrown nearly flat, the upward force from the water is enough to lift the stone back into the air before it has time to sink, allowing it to skip to the next bounce. Research has shown that the ideal angle for a stone to strike the water is close to twenty degrees; angles much steeper or much flatter than that both reduce the number of bounces a stone can achieve. Spin plays an important supporting role as well. A stone thrown with a fast backspin behaves more like a spinning top, staying level as it travels rather than wobbling or tipping to one side. Without enough spin, a stone is far more likely to catch an edge on the water and dig in rather than skip cleanly across the surface. Speed matters too, though less than angle: a faster throw simply gives the stone more energy to convert into additional bounces, as long as the angle and spin are already close to ideal.