A slight curve helps rocks make the biggest splash

Researchers were surprised to find that a very slightly curved object produces a more dramatic splash than a perfectly flat one.

When you want to find a rock that will make the biggest possible splash, you might do well looking for one that is ever so slightly curved. This will produce the largest impact force when it hits water, according to a finding that overturns a long-held belief in physics.

Flat rocks may not deliver the most dramatic splash
Julian Brooks/Alamy

The discovery was made by Jesse Belden at the Naval Undersea Warfare Center in Rhode Island and his colleagues, who set out to examine the idea that a flat object produces the greatest impact on water.

Instead, they found that a slight curve produced the largest effect. “It was kind of surprising for us to learn,” says Belden.

He and his team measured the impact force generated from dropping a half-kilogram aluminium cylinder from different heights into a fish tank-like container of water about a metre deep. The cylinder was fitted with various shapes of nose caps, ranging from perfectly flat to very curved.

As expected, they found that the perfectly flat nose cap produced a high-impact force. “But the surprising piece was that when we gave a slight positive curvature to the nose, extremely slight, that the force was even higher than if the nose was completely flat,” says Belden.

By using high-speed underwater cameras and complex calculations, the researchers learned that this was down to an air cushion that is produced at the point of impact by various shapes, softening the blow.

Very slightly curved objects produce a slightly thinner cushion than any other shape tested. This tiny variation has an outsized effect: it can increase the force generated on impact with water by more than 100 times compared with a perfectly flat nose and two times compared with a spherical nose.

“It’s essentially this really subtle difference in the air layer that’s on the order of tens of microns,” says Belden.

The finding could help engineers designing boat hulls minimise the impact force that a vessel experiences by avoiding shapes with the particular curvature found to make the biggest impact in the study, he says.

Journal reference:

 Physical Review Letters DOI: 10.1103/PhysRevLett.133.034002