Like Stones, Why Some Balls Bounce On Water

A Waboba is a ball that bounces on water (water bouncing ball, geddit?). That makes it kind of unusual since a simple experiment will show that many balls do not bounce on water. And that raises an interesting question–how does the Waboba work?

Today we get an answer from Michael Wright at Brigham Young University in Utah and a few buddies. These guys videoed the way three balls interact with water when bounced.

Here’s what they found. A Superball, which is solid and so has a relatively small surface area for its mass, burrows deep into the water, even when it hits at a shallow angle. So it does not bounce.

A raquet ball, on the other hand,is hollow and so has a larger surface area ratio to mass ratio. When thrown at a shallow angle, it penetrates only a small distance into the water creating a depression in the surface through which it planes back onto the surface. So it rebounds a little.

The Waboba is hollow but it is also soft and this turns out to be important. When it hits the water at a shallow angle, it too creates a bowl-shaped depression. But because it is soft, the ball flattens into disc-shape when it hits the surface and this allows it to aquaplane efficiently across the surface. And the angle of the bowl-shaped depression causes it to launch into the air where the ball regains its shape, making it look as if it has bounced.

The process is remarkably similar to the way stones skip across water, even though they are denser than the liquid. A shallow impact with the water surface creates a bowl-shaped depression that launches the stone into the air as it leaves.

Wright and co have filmed all these effects in a video placed on the arXiv today, another entry to this year’s Gallery of Fluid Motion run by the fluid dynamics division of the American Physical Society.

What’s impressive about the video is the simplicity of the experiments, many of which take place on a lake in a local park. And yet this belies the difficulties these guys must have had to deal with. Make no mistake, this kind of work is harder than they make it look. But then, that’s part of the art of film-making.

So why might this be useful? Wright and co don’t say in their video but that fact that one of the team is with the Naval Undersea Warfare Center in Newport might offer a clue.

Ref: arxiv.org/abs/1110.3989: Holy Balls!
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