An interdisciplinary team of researchers from King Abdullah University of Science and Technology in Saudi Arabia and Sofia University in Bulgaria are studying how objects float on the surface of water. They want to understand better how fluids and surfaces interact. Their research could help design better engineering systems for underwater vehicles and buoys.

The team published their findings in Physics of Fluids from AIP Publishing. They studied how round objects, like flat stones that you throw on the surface of a lake, move across the surface of water. They discovered that there are complex processes involved in how air moves around these objects that impacts how they move on the water’s surface.

How to skip rocks

Rock skipping is when a stone bounces on the water multiple times in a row, almost like it’s being supported by the surface of the water. This happens because the stone is actually colliding with the water very briefly each time it bounces.

The stone moves horizontally at a steady speed during each collision. With each skip, the stone is partly in the water, and the lift at the back of the rock makes it tilt and spin. The gyroscope effect of the rock’s spin helps to keep it stable.

Stones that are not facing the right way when they hit the water will not bounce. In fact, the largest angle at which a stone can rebound is about 45 degrees. A stone that hits the water at a 20-degree angle can still skip, even if it’s going slow, and it uses less energy to do it.

Why rocks skip

The study of fluids and physics with buoyancy involves some important principles: buoyancy, fluid motion, fluid resistance, and a Reynolds number (a dimensionless quantity that is used to predict the flow of fluids). Buoyancy is the force that pushes an object up when it’s in a fluid. Fluid motion is how fluids move and interact with solid objects.

Fluid resistance, or drag, happens when an object moves through a fluid and experiences resistance because of friction between its surface and the fluid. This resistance depends on things like how big the object is, what it’s shaped like, how fast it’s moving, and what the surface of the fluid is like.

Studying skipping rocks

The team found that when they threw the spheres harder and faster, the spheres moved around in a more irregular way. The spheres moved up and down, broke through the surface of the water, and made air pockets underwater.

The team also found that when they threw the spheres at different angles, the air pockets were different lengths, the spheres skipped different distances, and the spheres came out of the water at different times. The angle that they threw the spheres had a big effect on how the fluids and spheres interacted.

So what did we learn? Pretty much what we already knew. The size, shape, and angle of the throw matter more than how hard the stone is thrown across the water.