8/24 Skipping Stones

Q: Why is is possible to skip stones across water?

A: The reason that stones skip has to do with the speed, spin and angle of the stone as it hits the water.  If the stone is thrown properly it will hit with the front edge up, letting the back hit the water, and making a small dip.  The water will push back against the stone and the dip acts as a ramp for the front of the stone.
The spin provides stability and keeps the stone moving in the same direction, allowing it to skip again.  The angle that the stone is thrown at makes the biggest difference, the ideal angle is 20 degrees and any more than 45 degrees will cause the stone to sink immediately.  The spin needs to be about 5 rotations per second, but the higher the better and to create a force big enough to skip the stone has to be moving between 25 and 40 mph.  Technically anything can be skipped as long as it has a flat surface, but humans can't throw heavy stones fast enough to compensate for the added weight, so light ones work best. This is the equation for the skip of any given stone; it takes into account: diameter (), velocity (), mass (), tilt (), angle of attack (), and the density of water ().