November 30, 2021

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Physics of stones that jump over water

We all tried to make pebbles bounce on the water. Now, in the name of science, this has been done by a team of researchers from Chinese universities, who have carefully analyzed the phenomenon by publishing the results on fluid physics.

Gravity and spin launch. The general law is well known: the pebble is “pushed” up when it hits the surface of the water, and so, as long as the force of the lift is greater than the force of gravity, the stone continues to bounce and then sink. To be able to do this, the flat shape of the stone, the low angle of impact with the water and the impressive rotation of the launch is key.

Chinese researchers studied the details. Instead of an ordinary pebble, they used an aluminum disk with sensors that transmit data about rotation and movement. “Launcher” was a jet of compressed air with a mechanism capable of transmitting precise rotation of the disk. Moreover, the tracks and bounces were followed by video cameras placed above the water and at its surface level.

Space ship. Scientists have observed that the disc rebounds if the upward acceleration is sufficient upon impact with water: if the acceleration is more than four times that due to gravity (about 9.8 m/s2), the disc skips. If it is slightly lower (3.8 times), then the disc is “surfing”, that is, it glides over the water without jumping. The researchers also looked at the deflection effect of a rotating object moving in a fluid (Magnus effect): the disc pebble does not travel in a straight line, but rather bends in the direction of rotation, and it bends more and more with each rebound. Physicists hypothesized that their research could be useful in designing boats or submarines, or for re-entry of spacecraft on water.

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