Max Lin '24
Physics Behind Rock Skipping
Since I was young, rock-skipping has always been my most favorite hobby, after I was taught by my dad. Whenever I go to a river, lake, or anywhere with a big expanse of water (and suitable skipping rocks scattered on the ground), I will skip rocks. However, rock-skipping is not just throwing a rock randomly and trying to make it skip. It involves some physics too, which can also calculate how many skips you would make. The basics of rock skipping are this: pick a flat, round rock that shouldn’t be bigger than the palm of your hand, hold it with your index finger on the top edge, thumb on top, and middle finger on the bottom, close to the thumb’s position with fourth and pinkie fingers supporting the 3rd finger (this is just one of many ways to hold the rock for skipping!). Then, you whip your arm sideways, causing the rock to be thrown while spinning, at a very acute angle to the water, and watch it skip!
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There are many factors that contribute to a rock skip and determining how far it can go, and how many times it will skip. First factor is the attack angle, which is basically the angle at which the rock is tilted compared to the surface of the water. Next is the course angle, the angle at which the rock is thrown into the water. Afterwards is the amount of rotations per minute the rock has (rpm), impact velocity, and a hydrodynamic pressure force normal to the stone’s broad surfaces that then jettisons the stone. Here is a figure that shows all of the variables:
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Alpha is the attack angle, beta is the course angle, F is the hydrodynamic pressure force, and U is the impact velocity. S is the area on the rock that is wetted by the water. Now, how do all of these variables come together to make the rock skip? When the rock is thrown, spinning, it (should) be slightly tilted upwards, with the course angle being about 20 degrees (a value determined by many experiments to be the optimal course angle of the rock as well as the attack angle). When the rock hits the water, it creates a sort of small dent, and the wave made by the rock helps the rock to be propelled forward using the rock’s velocity. All while it is skipping, the spinning of the rock helps to gyroscopically stabilize the rock, so the attack angle can be maintained. Usually, a right balance between power and spin has to be made, as sometimes spinning the rock more sacrifices the amount of power you put into the throw, which can affect how hard you throw it, contributing to the end result of skips/distance you get. Couple of fun facts: did you know, in naval warfare during WWII, a British scientist named Barnes Wallis designed a bouncing bomb that would be skipped like a rock across water to breach German dams? In addition, the record for the most amount of consecutive rock skips was set by Kurt Steiner, in September 2013, with an insane amount of 88 SKIPS. If you want to witness the skipping spectacle, you can look up “rock skipping record” on YouTube and you should find it. For comparison, my record is about 25, and I would say most professionals skip about in the 40s, maybe 50s if they are lucky, so 88 skips is absolutely crazy.