By Matthew Sparkes, newscientist.com
April 20, 2021
People have been skipping – or skimming – stones for thousands of years, but physicists are continuing to learn more about the phenomenon. The latest insights suggest new techniques that could help bring spacecraft safely back to Earth.
A team involving researchers from several Chinese universities made an experimental model of a skipping stone using an aluminium disc. The model contained electronic sensors to transmit details of its spin and movement to a computer via Bluetooth.
The aluminium disc was then launched towards a pool of water using a puff of air and a motor that gave it a precise rate of spin, which previous research has shown is key to getting a skip. A high-speed camera above and another positioned parallel to the water surface tracked the disc’s movement in two planes.
The team found that the disc either bounced or surfed on the water depending on its rate of acceleration when it first struck the water. When acceleration exceeded four times the acceleration due to gravity, the disc bounced. When acceleration was slightly less, at around 3.8 times, the disc surfed without bouncing.
The researchers also learned that the Magnus effect, which causes an object to deviate from a straight line when spinning, played a role both during flight and while bouncing. The path a stone takes as it bounces across water isn’t straight, but curves towards the direction of the spin, and the amount of curve increases with the number of bounces.
The team says that the lessons learned can help engineers build better boats, submarines and seaplanes, suggesting the results could even help inform the design of spacecraft for bringing astronauts to Earth by landing in the sea.
Maarten van Reeuwijk at Imperial College London says there is still more to explore concerning the physics of stone skipping, but he says that the paper is a “fairly complete model”.
However, he believes that suggestions the research could lead to safer water landings for astronauts returning in a spacecraft is problematic. “It would need to be spinning very fast, so having any humans in there is not a good idea,” he says. “You imagine spinning through the entire re-entry phase. But [uncrewed] could work. You need a low angle of attack, you need to be spinning very fast, and I can imagine that it’s very tricky. And you might need so many boosters and adjustments that it’s easier just to land the thing like SpaceX is doing.”
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