Making stones skip across the surface of water is an art, but it's also increasingly turning into an area of serious scientific inquiry. Splash Lab at the University of Utah studies fluid dynamics (the branch of physics that deals with the motion of liquids and gases) and researchers there have been looking into what it takes to get a good bounce on water.
Many variables influence what happens when an object hits water: the angle of impact, the shape of the object, its rotation, size, mass and coefficient of elastic deformation (like is it hard or soft?). Splash Lab researchers study all these variables with high-speed cameras capturing 1,000 images per second, permitting them to see what goes on at the exact moment of the bounce.
The photo above shows a ball right after a bounce. The big air cavity in the water was caused by the impact, and because the projectile had so much momentum, it rode up the side of the cavity (from left to right in this image), which acted as a ramp, making it jump out of the water on its way to the next bounce.
This 2012 video by Splash Lab goes into further detail about what it takes for a good bounce and how the lab studies fluid dynamics:
The stone-skipping record mentioned in the video has since been broken: In 2014, Kurt Steiner established a new record with 88 skips. (Coincidentally, “steiner” means “stones” in Norwegian.) His feat was captured on this amazing video:
Research into stone-skipping might seem unusual, but splash science can lead to real applications in various fields like energy, transportation and medicine. For example, Zodiac-style inflatable boats could be made more fuel-efficient and safer for passengers by better controlling how they bounce on the water.
Recently, Splash Lab and its collaborators have been looking into the water-skipping properties of the elastic sphere, which in many ways is better for the task than the traditional rock. Through trial and error, the lab discovered that more squishy spheres had the best skipping potential and could even skip when thrown from angles that were three times as large as the maximum angle for a rock. (Research published in 2004 has shown that the ideal angle for a stone is 20 degrees.) This might seem surprising since the best water-skipping rocks tend to be flat and hard, the opposite of a round and soft ball, but it makes sense when you take a closer look. The crucial moment is when the ball hits the water. It then “naturally assumes disk-like geometry and dynamic orientation that are favourable for water-skipping,” the researchers write in Nature.
Here you can see an elastic sphere shot out of an air cannon doing an impressive series of controlled bounces across multiple water containers positioned across the lab:
How to make stones skip on water like a pro
If you want to try your hand at a new stone-skipping record, here are a few pointers from Alessandro Gonella, a self-proclaimed stone-skipping expert. As you can see in the video, he’s exceptionally good.