Balls jump like stones on the water

Fluid physics: why balls jump over water so well

Who can do more jumps? Letting stones bounce over the water is an art in itself: the pebbles have to be the right size and shape - and hit the pond or river surface at the right angle. How much easier is it to sit down with a plastic ball that, regardless of how it was thrown, jumps over the water more often than not. But why? To do this, Tadd Truscott from Utah State University and his team let elastic and rigid bodies fly over liquids several times and filmed the whole thing with high-speed cameras. The evaluation confirmed earlier studies on the one hand: the hard stones had to have a certain shape - disk-like pebbles - and be thrown at a perfect angle or hit the surface. Only a few degrees of deviation ensured that the projectile sank immediately.

The elastic balls, on the other hand, made the trick easier: when they hit the water, they deform in the direction of a disk-shaped geometry and thus resemble perfectly shaped pebbles. In addition, the deformation ensures that the water exerts a stronger lifting force on the ball than on non-deformable stones. At the same time, thanks to the deformation, the elastic objects can hit the liquid from steeper angles and bounce more often than solid materials. Depending on the angle of throw and impact, the acceleration and other parameters, the researchers can even calculate how often, for example, a ball can sit down before it sinks or swims away. This research not only has a playful aspect, says Truscott, but also serves materials research. The findings could not only be used to develop new water toys, but also plastic lifeboats that are difficult to sink.