FEB 06, 2016 7:42 AM PST

The Science of Skipping Spheres

WRITTEN BY: Cassidy Reich
In a recent paper whimsically titled “Elastic spheres can walk on water,” a group from Utah State University have empirically shown why it is easier to skip an elastic sphere (such as the water toy the WaBoBa) on water compared to a rigid sphere.

This kind of research does actually have a lot of practical implications beyond being a fun thing to do at the lake. Inflatable boats exhibit different behavior on water compared to a boat made of more rigid material, and it is important to understand why. Specifically, the Navy thinks it is important to understand why considering they were the ones who funded this research.

The physics of how a stone skips on water have been well-studied. Briefly, that phenomenon can be described as “an object obliquely impacting a water surface with sufficient inertia will carve a cavity on the air-water interface and experience a pressure-driven hydrodynamic force dependent on object velocity, geometry, and orientation.” That is a very science-y way of saying what you’ve probably figured out through trial-and-error: the stone needs to be kind of flat and thrown in the right way (more out, less down) and at the right speed to skip.

To gt a rigid sphere to skip (which is much harder than the favored flat stone), the impact angle on the water has to be below an upper limit, termed βo. An elastic sphere’s βo is up to three times larger than a rigid sphere’s so there is a much larger window of acceptable impact angles to get an elastic sphere to skip. This expanded βo (plus a lot of other intense mathematics) means that elastic spheres can do multiple skips over long periods of time.



In addition to tolerating higher impact angles, elastic spheres actually deform upon impact with water, giving them the more favorable disk shape that we all know works better for skipping rigid stones. In this slow-motion clip of an elastic sphere hitting the water, you can see how it flattens out on impact.



Hilariously, this line of research got started simply because the senior researchers’s son and nephew wanted to see a slow motion video of a rubber ball skipping on water. That curiosity led to important and translational discoveries on the way elastic objects interact with the surface of water.

Sources: EurekAlert and Nature Communications
About the Author
  • Cassidy is a curious person, and her curiosity has led her to pursue a PhD in Pharmacology at the New York University Sackler Institute of Biomedical Sciences. She likes to talk about science way too much, so now she's going to try writing about it.
You May Also Like
MAR 27, 2020
Chemistry & Physics
MAR 27, 2020
Hand Sanitizer Shorage: Liquor, Perfume, and Medication Producers Joined the Race to Restock the Shelves
Amid the coronavirus pandemic, the habit of thorough and frequent handwashing are among the best defense against the con ...
MAR 31, 2020
Chemistry & Physics
MAR 31, 2020
Pandemic in Silico: How Maths Modeling Helps Our COVID Fight
The phrase "flattening the curve" is used frequently these days by epidemiologists to describe various measure ...
APR 20, 2020
Space & Astronomy
APR 20, 2020
Here's How the ESA's CHEOPS Mission Works
Exoplanetary detections have exploded exponentially since the first discovery of one by Swedish astronomers in 1995. The ...
APR 29, 2020
Chemistry & Physics
APR 29, 2020
New method hospitals can use to produce hydrogen peroxide
In a collaboration between the University of California San Diego, Columbia University, Brookhaven National Laboratory, ...
MAY 19, 2020
Space & Astronomy
MAY 19, 2020
The Science Behind Eclipses
A particularly convenient coincidence exists between the relative sizes of the Sun and the Moon, and their distance from ...
MAY 21, 2020
Chemistry & Physics
MAY 21, 2020
The Nature of Glass Still Dumbfounds Scientists
There are many things we humans have come to perfect, but don't yet fully understand. Take glass for example, scientists ...
Loading Comments...