Above: Image © Andreya , iStockphoto.com

Ever stood by the water and skipped stones? Did you then challenge your friends or family for the title of champion stone skimmer? If you're really serious about this sport, then maybe you should try for the world championships!

The annual World Stone Skimming Championships takes place on Easdale Island near Oban in Argyll, Scotland. It's a free event, and everyone is eligible to participate. Easdale Island has a large abundance of slate - a sedimentary rock which splits into flat plates — perfect for stone skimming!

Did you know? The ideal rock for stone skimming is: round, flat, fits in your palm, weighs enough so wind does not affect its flight, and light enough so that it can be thrown without difficulty.

For anyone who has tried skimming stones, they know that to skim a stone, it should be thrown with a fast horizontal velocity, with a spin like a Frisbee, and it should be thrown from the edge of the water approximately 10 to 20 degrees to the water. So what makes stones thrown this way bounce but others that are thrown differently sink? The answer this we first need to discuss momentum and its conservation.

Did you know? Momentum is a measure of how difficult it is to stop an object. Its value is the mass of the object multiplied by its velocity.

To understand momentum, imagine a soccer ball and a bowling ball both moving at about the same speed. Now try to stop both balls. The soccer ball should be relatively easy to stop, while the bowling ball is difficult to stop. This is because the soccer ball has less momentum (due to its lesser mass) than the bowling ball.

The law of conservation of momentum is a fundamental law of nature and states that the momentum of a system remains constant. For example, when a gun fires, the momentum of the gun must cancel the forward momentum of the bullet so the gun recoils (i.e., go backwards). Now back to the need for horizontal velocity... When the stone hits the water, water gets pushed downwards, and the water gains a downward momentum. Due to the law of conservation of momentum, the stone now gains momentum in the opposite direction. If there is enough water displaced downwards by the stone, the stone will be pushed back hard enough to bounce up into the air!

But there's a problem (at least for stone skippers). The stone that drops straight down will only displace a smaller amount of water. Although the resulting upwards momentum would slow the decent of the stone, it won't be enough to pop the stone back into the air. In order for that to happen, more water needs to be pushed down, but this doesn't occur because the water, being a liquid, will be displaced around the stone.

On top of that, any water ending up on top of the stone will push down on the stone — canceling any upwards force of the water below the stone. Once this happens the stone, being heavier than water, is doomed to sink.

Did you know? The current Guinness world record holder for stone skipping is Kurt Steiner of Emporium, Pennsylvania. His record breaking throw produced a rock that skipped 88 times!

To over come this, stone skimmers must rely on angular momentum. While this is not an absolute condition for skimming, the spin is important to help maximize the number of skims possible. When something is spinning, like a Frisbee, it has an angular momentum, which is really just momentum applied rotationally.

Things with angular momentum will resist spinning in other directions. So, angular momentum gives the stone stability and ensures it will continue to travel parallel to the water. This in turn maximizes the surface area of the contact with the water, so the stone is able to push down more water. If instead the stone hit the water at a sharp angle, then the amount of water displaced may not be enough to meet the requirements of skimming.

Learn more!

How to Skim Stones

The Mystery of the Skipping Stone

World Stone Skimming Championships

Conservation of Momentum

Angular Momentum

 

Tony Lim

I am a graduate student pursuing a PhD in the neuroscience program at McGill University. In addition to neuroscience, my scientific interest and background lies in the field of pharmacology. When not at the lab, I enjoy playing volleyball, badminton, snowboarding and playing electric guitar with the band in my spare time.


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