Stone skipping

The skipping stones, colloquially Ditschen, stone rapids, Pfitscheln, Steineflitschen, flick, cliffs, Platteln, slates or Austrian also called " flacherln " is a pastime which was operated in ancient Greece and described by Homer. Goal is a flat stone to hurl so that he often possible to jump over a water surface before it sinks. Instantaneous world record holder in the Guinness Book of World Records with 51 jumps is the American Russell Byars.

Physical Basics

Anyone who has ever tried in skipping stones, realizes that he has to fulfill some physical conditions for perfect control. Stone not only has the shape of a flat ellipsoid or disk have but also thrown so that the flat side being parallel to the water surface. The discharge height should be as low as possible, preferably not much higher than the water surface itself is also required is wave low, quiet waters as well as little crosswind. In addition, the stone in rotation must be rotated about its vertical axis. This behavior of gyroscopes is known, as long as not disturbing the movement of the torque acting on the body, the rotation remains unchanged and stabilized missile. If you throw stones without this additional angular momentum, so they lose by small disturbances during flight their orientation. Then when hitting the water, they dive under. An intrinsic angular movement of the stone to reach, by holding the stone between thumb and forefinger and exercises at the moment the goalkeeper on the edge of the stone with the pointer finger.

Once the stone bouncing on the water surface, however, he does not jump, as one might think at first, like a ball back, because the water surface does not act like a solid body. That is why it is amazing that stones can ever jump on water. Show footage that dropped at an acute angle to the water surface stones put first on the water with its rear edge. The stone slides then stabilized by its rotational movement, initially a small piece on the water surface and pushes a small water wall like a bow wave in front of him, which he, at sufficient speed catches up: How to a jump he slides up to this wave and sets to the next jump. Friction losses of course, he loses all contact with the water surface both motion and rotational energy. The jumps are thus increasingly shorter and then go into a kind of skidding. Finally, either the speed of the stone is so small that it can not catch the bow wave and sinks into the water, or twist enough - this is especially the case with small stones - the web stabilization no longer sufficient. The stone then no longer applies flat on the water and dives.

Researchers from the University of Marseille and Lyon have studied the conditions for the perfect stone 's throw experimentally. They constructed a catapult that hurled aluminum disks as flat model stones on a water surface. In the throws, the researchers varied the initial velocity of the stone, its impact angle on the water as well as the self-rotation of the disc. The motion was documented with a high speed camera. In evaluating the data, the researchers came to the conclusion that short contact times with the water surface significantly affect the number of possible jumps: The shorter the contact, the less energy is lost through friction. In the experiment this time was less than 1/100 second. The energy losses are also the reason why stones with small initial velocities are very successful. Independently of the self-rotation or the speed of the stone, the optimum contact time has been reached when the stone hit at an angle of 20 degrees to the water surface. At impact angles above 45 degrees, the jumping phenomenon could not be observed.

Also on wet sand can be achieved stone cracks. Here, one can observe that both short and long jump distances alternate. Show film footage that the short distances arise when trailing and leading edge of the stone striking the firmer compared to water sand. The stone is thus slowed down by the impact so that it tilts before it attaches again to jump.

Re-entry into the atmosphere

This bouncing effect is often taken as an explanation for the " bounce " of a spacecraft to the shallow re-entry into the Earth's atmosphere. However, this is wrong, as usual re-entry for it to generate enough lift. The " bounce " is a geometric effect: to slight braking the train remains approximately an ellipse, on which the body first approaches the planet and later removed. If this path is interpreted as a height above the planet's surface, initially results in a decrease and later an increase. Also a multiple dipping into the atmosphere at one atmosphere stunt shows with a simple plot of the orbital altitude a similar picture as the jumping stone, but has a very different cause. Spaceplane, with much more buoyant as the silver bird or the Wave Rider would make a hopping similar to the stone in the upper atmosphere possible, but are still in the future.

History

Formerly known oyster shells were used for " Ditschen " what a historian documented already in 1583. In Homer's tale of Hercules and Jason throw their shields over the water. Shakespeare mentioned in the original version of " Henry V " the word "stone - skipping ". Inuit and the Bedouins know the game well and use ice or sand as a substrate.