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Question How does a stone "skip" across water?

Answer

Spin, speed, shape and angle are the crucial factors, with angle being the most important.

Skipping stones at Seneca Rocks.  Ryan Hagerty, USFWS photographer, 2014. U.S. Fish and Wildlife Service National Digital Library

Spin stabilizes an object and keeps it from simply falling into the water. A minimum speed must be achieved or the stone will hit the water and sink immediately. Flat, round stones are best because the surface area creates a bounce on impact, but the “magic angle” between a spinning stone and the water should be about 20 degrees in order to achieve the maximum number of skips (Clanet).

According to Jerry McGhee, founder of the North American Stone Skipping Association (NASSA), both Shakespeare and Homer mentioned stone skipping.  Eskimos skip rocks on ice and Bedouins on smooth sand.  In England, stone skipping is known as “ducks and drakes,” in France, as “ricochet,” in Ireland, as “stone skiffing,” in Denmark as “smutting,” and every language, from Hindi to Russian to Chinese, has a unique word or term for skipping stones.

A word used in American English is “to dap” defined by the Oxford English Dictionary as “to rebound, bounce; to hop or skip (as a stone along the surface of water).”

Rocky Beach with a close view of stones.  Noatak National Preserve, Alaska, 2013.  U.S. National Park Service, NP Gallery

We can learn more by applying concepts from physics, such as hydrodynamics, momentum, and gravity.  The basic physics of stone skipping has been understood through the use of laboratory equipment especially designed to skip stones. The equipment records the motion using video and digital photography.  Observations revealed that while it is possible for one skip to be longer than the previous one, possibly due to an uneven water surface, the distance between each skip is usually about 80 percent of the previous skip.  Another interesting observation shows that, for right-handed throws, the later skips veer to the right.

On a weekend trip to Acadia National Park in Maine, the President showed his daughters, Malia and Sasha, how to skip stones during a hike in the park.  Pete Souza, White House photographer, 2010. White House Archives at the National Archives and Records Administration (NARA)

A stone’s travel through the air is considered ballistic, defined as “relating to or characteristic of the motion of objects moving under their own momentum and the force of gravity” (http://wordnet.princeton.edu external link).  A stone’s interaction with the surface of water, however, is different. Each time the stone skips the surface of the water it is reflected upwards, its downward velocity is reversed, and its horizontal velocity is reduced. Since the trailing edge of the stone typically breaches the water first, it is also pitched down slightly.  This brief downward pitching affects the direction of the stone’s path.  Each subsequent bounce slows it down until it penetrates the water surface rather than skipping over it.  The height from which it is thrown, the angle, the impact attitude, and the condition of the water’s surface – all are additional factors that affect how many skips occur and how quickly the stone splashes down.

Skipping a stone into the Kawishiwi River.External link  Daakrolb, photographer, 2010.  Wikimedia Commons

The principle of the conservation of momentum dictates that as the stone enters the water and pushes some of the water downwards, the stone is forced upwards.  This force is equal to the hydrodynamic pressure on the stone multiplied by its area.  Assuming that this force is balanced against the weight of the stone, then Mg, where M is its mass and g is the acceleration due to gravity, there is a minimum velocity—a few kilometers per hour—above which the stone will bounce.  In other words, a stone has to have a minimum velocity in order to bounce.  If its velocity is less than this value, the stone skims across the water for a short distance and then sinks.

What is the Record for the Highest Number of Skips?

Previously, the record for the highest number of skip was 40, set by Kurt Steiner at the Pennsylvania Qualifying Stone Skipping Tournament on September 14, 2002 (Guinness Book of World Records).  In 2007, however, Guinness confirmed that Russell “Rock Bottom” Byars broke the record with 51 skips.

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