Why do stones skip across water

According to Bocquet, a stone will only bounce if its initial velocity exceeds a certain value. If the stone is also spinning, this introduces a stabilizing torque that can maintain the initial angle at which it hits the water — which helps the stone bounce again. The maximum number of bounces depends on the rate at which the stone decelerates — which is in turn directly related to its initial velocity. In principle, a stone could be made to bounce many times by increasing its initial velocity.

In practice, however, the number of bounces are limited by the angular destabilization factor — which is independent of the initial velocity. Ultimately he hopes that his calculations will allow someone to break the world record of 38 bounces which, if Bocquet is right, is achieved by throwing the stone at 12 metres per second with an initial spin of 14 revolutions per second. Close search menu Submit search Type to search.

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After experimenting with marbles, catapults, and tubs of water, Wallis developed cylindrical bombs that could be spun while airborne, then released to skip across the water. The first bomb detonated before reaching its target; the second hopped over the dam and destroyed the power station below it; the third veered to the left; and the fourth struck home, reaching the dam after three bounces.

More recently, engineers at Lawrence Livermore National Laboratory have proposed the HyperSoar airplane, which would skip along Earth's upper atmosphere at five to 12 times the speed of sound. The plane would take off like a regular passenger plane, soar 25 miles above Earth, then kill its engines and drift up into space and slowly back down again.

When it hit the denser air of the upper atmosphere, it would bounce back up like a stone hitting water. The pilot would then fire the engines again and send the plane back into space. Eighteen skips would be enough to get HyperSoar from Chicago to Rome in 72 minutes. McGhee isn't surprised by that, but he doubts that mathematics will ever grasp what's involved in a truly great throw.

When all goes just right, a stone seems to dance across the water forever, its skips almost too quick to count. Pick a smooth stone that fits in your palm, has a uniform thickness, and is neither too heavy nor too light.

McGhee likes ones that are about as heavy as tennis balls. Rectangular stones will veer off at an angle; triangular stones are the most stable on choppy water. Irregular stones are McGhee's favorites. Stand at an angle to the water, your feet apart at shoulder width. Take a few warm-up throws with increasingly heavy stones. When you're ready, gather some stones and hold them in your non-throwing hand as a counterbalance. Then place one in the crook of your index finger and cock your wrist.

Breathe in slowly through your nose, extending your arm high above your head. The question is, why is twenty degrees the best angle? Sure, stones will skip if they come in at a shallower angle, but not as far.

Chuck them in at an angle of greater than forty-five degrees and they'll just sink, sullenly into the water. This has to do with the mechanics of the skip. Anyone who has practiced skipping stones across the water knows that the only stones that work are ones that are relatively light weight, and flat stones.

The light weight part is obvious. A person would have to heave a pyramid block at a very precise twenty degrees to get it to skim across the ocean like a flying fish. Around , for instance, mathematician William Bourne noted that cannonballs fired from ships at a sufficiently low angle could ricochet across the water's surface, bouncing onto decks and breaking masts on the the target ships.

And during World War II, British engineer Barnes Wallis came up with a "bouncing bomb" design , in which the weapon bounced across the water before striking the target, then sank and exploded underwater, akin to a depth charge.

The Royal Air Force used bouncing bombs against Germany in More directly relevant to the current paper, in , Theodore von Karman conducted several experiments to determine the maximum pressure on seaplanes during water landings, and in , Herbert Wagner showed that the takeoff and landing of a seaplane was essentially all about impacts and sliding on a liquid surface.

For their new research, the Chinese team focused on bouncing skipping and surfing, in which the disk or stone skims the surface and never bounces.

The researchers came up with their own theoretical model of the phenomenon that incorporated not only the aforementioned gyroscopic effect, but also the Magnus effect. It's long been known that the movement of a baseball , for instance, creates a whirlpool of air around it. The raised seams churn the air around the ball, creating high-pressure zones in various locations depending on the type of pitch that can cause deviations in its trajectory.

Something similar occurs with skipping stones. To test their model, the Chinese scientists created an experimental setup involving a flat aluminum disk and a launching system with a brushless motor to ensure the disk could reach the necessary speeds. The launching system used puffs of air from a compressor to control the disk's speed as it traveled toward the water. The researchers attached a nylon cap to the disk, connecting it to the launcher via a magnetic base.

The cap also held an inertial navigation module to measure and collect the data during launch, "flight," and landing, transmitting that data to a computer via a Bluetooth connection.

The team found that the critical threshold for vertical acceleration is four times the acceleration due to gravity 4 g.