FYFD

Almost everyone has tried skipping rocks across the surface of a pond or lake. Here Professor Tadd Truscott gives a primer on the physics of rock skipping, including some high-speed video of the impact and rebound. In a conventional side-arm-launched skip, the rock’s impact creates a cavity, whose edge the rock rides. This pitches the rock upward, creating a lifting force that launches the rock back up for another skip. Alternatively, you can launch a rock overhand with a strong backspin. The rock will go under the surface, but if there’s enough spin on it, there will be sufficient circulation to create lift that brings the rock back up. This is the same Magnus effect used in many sports to control the behavior of a ball–whether it’s a corner or free kick in soccer or a spike in volleyball or tennis. (Video credit: BYU Splash Lab/Brigham Young University)