Jumping rocks over a body of water is an age-old game, but developing a better understanding of the physics involved is crucial for more serious issues, such as landings on water when space flight vehicles re-enter or d ‘planes.
In Fluid physics, by AIP Publishing, scientists at several Chinese universities are revealing several key factors that influence the number of bounces that a rock or a landing plane will experience when hitting the water.
The study involved theoretical modeling and a simple experimental setup using a model stone to collect data in real time. Investigators used an aluminum disc as a substitute for the stone and devised a launching mechanism that used a puff of air from a compressor to control the speed at which the disc was moving toward the water.
Previous studies had already determined that spinning the stone is the key to making it jump or bounce, so the experimental setup allowed a motor to apply controlled rotation to the disc before launch. In addition, the disc had a nylon cap containing an inertial navigation module to measure the data in flight and transmit it to a computer via a Bluetooth connection.
Investigators observed two types of movements after the disc collided with the water surface: rebounding and surfing. In the latter, the disc travels the surface of the water without bouncing at all.
A key amount in determining whether the disc can rebound is vertical acceleration. When this acceleration exceeds four times the acceleration due to gravity, g, the disc rebounds. When it is slightly smaller, 3.8 g, it was observed surfing.
“We see the surf phenomenon as a critical form of rebound, with 3.8g as the critical rebound limit,” said author Kun Zhao. The minimum value at which the stone has the potential to jump turned out to be 3.05 g.
Scientists have also found that the direction in which the disc or stone is turned affects its trajectory and attitude or pitch, which is the angle between the surface of the water and the direction of flight.
“Our results show that the main effect of the rotation is to stabilize the attitude during the collision by the gyroscopic effect,” Zhao said.
The rotation also deflected the flight path of the disc in flight. A clockwise rotation bent the path to the right, while a counterclockwise rotation deflected it to the left.
“Our results offer a new perspective for advancing future studies in aerospace and marine engineering,” Zhao said.
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