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The world record holder at skipping rocks kinda sucks at skipping rocks, so do you and so does…well, everybody. Not to come screaming in with a hot take about a relaxing outdoor pastime with zero stakes, but it’s true. While the world record is 88 skips in a row, researchers predict we should be able to do hundreds, but we can’t.
In our quest to understand why, researchers have uncovered some real-world applications, including bringing astronauts safely back from space. Listen, I’m pretty stoked if I can get three skips, so 88 seems unfathomable. But from what we know of the physics behind rock skipping, it’s very possible.
The throw itself involves two things: speed and angles. First, imagine the throw. The stone has to be traveling quickly in order to have enough momentum to keep going after making contact with the water, and it’s not just traveling really fast in a straight line. The stone is angled slightly in order to collide with the water. On top of that, the stone needs to be rotating, which you accomplish by holding the stone with your forefinger on the leading edge as you throw. This rotation is part of the key to success because it gives the stone stability as it flies through the air.
As the stone collides with the water, there are a couple of forces acting on it. The water pushes up on the moving stone as it makes impact, causing the stone to skip. The collision also causes the water itself to move, creating ripples. These ripples will add some drag and slow its momentum. You’ll notice that as the stone begins to lose momentum, it starts skipping more frequently, and the height of its skips decrease until all of a sudden the fun is over. Fun fact: professional skippers call these frequent skips a “pity patch”.
So, if the goal is to maximize your skips and get somewhere in the same ballpark as the world record, you have to do a few things to overcome the forces acting on the stone. You need to pick a flat, lightweight stone, so the upward force of the water is greater than the weight of the stone, otherwise it will just sink as soon as it hits. In a 2004 Nature paper (yes, a Nature paper about skipping stones), the authors identify the magic angle of attack when throwing your stone. They found that using a 20-degree angle in relation to the surface of the water maximizes the number of skips and minimizes the amount of energy lost, keeping the stone skipping longer.
Even with all the forces acting against the stone, models like this say we could create even more skips. But why is this so hard to put into practice? First, it’s really hard to throw fast with spin while at the same time being very accurate about hitting the magic angle of attack. At that point, you’re talking major league baseball pitcher levels of power and precision, and most pitchers don’t moonlight as stone skippers.
But even if you could achieve that combination, there are a lot of other factors working against you. For one thing, Mother Nature didn’t make all stones perfectly flat and round discs. And researchers say that the best skipping stones aren’t stones at all. If you could, you’d use something a bit squishy that deforms as it connects with the water, like a rubber ball – but that’s littering, so don’t do that.
Also, most models simplify the real-world environment, neglecting things like wind and waves, all of which act against the stone. So, while way more than 88 skips could be possible in reality, there are too many factors that are out of our control to make that happen without the help of a controlled environment – and maybe a robot.
But in the pursuit of trying to understand skipping stone physics, researchers have found some real-world applications. For example, a skipping technique was applied to the Orion capsule that landed in December 2022. NASA’s Artemis program recently achieved its first successful uncrewed mission with Orion. When it came time for Orion to return to Earth, it skipped off the atmosphere before re-entry. This maneuver helped the spacecraft land more accurately, and also slowed the speed of the spacecraft and reduced the amount of heat generated by the high speed entrance. While Orion was the first to successfully skip across Earth’s atmosphere, the Apollo missions had the capability to do the same, though not the confidence.
The physics behind a simple game like skipping stones can teach us a lot about energy momentum conservation. To learn more about this fascinating topic, check out Brilliant.org’s SciShow course on special relativity taught by a Penn State University PhD in physics. The course is even turned into a YouTube video series by Minute Physics. With 18 lessons in special relativity, you’ll be ready to tackle Quantum objects and gravitational physics.
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