For those of you who have been fascinated with skipping stones, this is a great video. We actually see how a projectile hits the surface of a liquid, scoops out a hollow, and bounces back upwards. Take a closer look at a skipping stone in action.

That videos hows exactly what physicists have observed in skipping stones. The back edge scoops through the water. (On more solid surfaces, when a stone hits the surface with its back edge, it then faceplants, falling forward to hit with its front edge.) We can see that the stone does move back up, but the motion still looks uncanny, if not wrong. How do stones skip? The best explanation I’ve found is at Everyday Mysteries, a site hosted by the Library of Congress:

“The principle of the conservation of momentum dictates that as the stone enters the water and pushes some of the water downwards, the stone is forced upwards. This force is equal to the hydrodynamic pressure on the stone multiplied by its area. Assuming that this force is balanced against the weight of the stone, then Mg, where M is its mass and g is the acceleration due to gravity, there is a minimum velocity - a few kilometers per hour - above which the stone will bounce.”

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Here’s another shot of a skipping stone. This one struggles a little.

Top Image: Killy Ridols

[Souces: Annual Reviews, Everyday Mysteries.]

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