The icy ballet of stone upon ice makes curling one of the most intriguing sports of the Winter Olympics. But scientists still haven’t quite figured out the physics of how they move.
For a sport that dates from the 1500s, it’s surprising to learn that we’ve still not worked out how curling works.
The Winter Olympic sport – first played on frozen lochs in Scotland centuries ago – involves launching hunks of granite across a rough icy surface, so that they drift and bend into a target known as the house. Players sweep brooms in front of each stone to control its trajectory.
This seems simple enough, but as curling coaches themselves acknowledge, there are still many unknowns about the physics of the sport. So while the basic tactics are agreed, there are often divides and controversies about the most effective techniques, even at a high level.
Of all the scientific mysteries, one of the biggest speaks to the name of the sport itself: how and why do the stones curl? If a player spins a stone clockwise at the moment of launch, it will curl to the right towards the end of its journey, or vice versa. With a cursory knowledge of physics, this is not what you might expect. You can see why for yourself if you launch a spinning bowl or upturned glass over a carpet or rug: it will curl the opposite way it’s spinning. Why don’t the stones do this?
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“The scientific community hasn’t come to a consensus on the physics of curling, although it’s not through a lack of effort,” says Jennifer Vail, author of Friction: A Biography, who specialises in “tribology”: the scientific study of friction, lubrication and wear. “It’s been over a 100 years since researchers started trying to understand it, but the mechanisms behind the curling of the stone remain unsolved.”
For starters, the granite stones, despite their appearance, are not just any blocks of rock. Mined from only two places in Wales and Scotland, they are particularly tough and water-resistant. And the shape is key: the underside is concave with rims called “running bands”, similar to the base of a beer bottle. It’s this band that makes contact with the ice.
The ice is specially designed too. Unlike a typical ice rink, the surface is “pebbled” before matches, with tiny droplets of water sprinkled above to create a rough surface. “Without that pebbling, friction would prevent the stone from ever reaching the house,” says Vail. “This may seem counterintuitive since bumpy roads slow us down when driving but for curling, those pebbles reduce contact between the stone and ice, which reduces friction.”
Then there’s the effect of water. As the stone slides, “the ice heats up enough to melt and create an incredibly thin layer of lubricating water”, says Vail. “This lowers the friction, helping to keep the stone moving while also influencing the trajectory of the stone.”
Research in 2024 explored the three phases of the stone’s journey across the ice. After the stone is launched, at its highest speed, the higher volume of friction-induced meltwater means it effectively “hydroplanes” in a straight line.
Sweepers extend that distance by rubbing their brooms on the ice surface ahead of the stone, creating extra water lubrication. As it slows down, the water lessens and the abrasion of the hard ice starts to kick in; this is where it begins to curl. Finally, as it slows towards a stop, the water disappears entirely and it experiences totally dry friction, slowing to a stop.
As broom technology has advanced, so too have the sweeping methods
Players use different techniques to try and control the curl, such as sweeping on one side of the stone as it slides, or sweeping directionally at an angle, but scientifically it’s often not been clear why some techniques work over others.
As broom technology has advanced, so too have the sweeping methods. Sometimes this leads to rule changes. In 2015, new brush materials capable of scratching the ice seemed to give some players an unfair advantage. A technological doping scandal known as “broomgate” followed, prompting the World Curling Federation (WCF) to ban certain brush-types in 2016, only allowing a standard smooth nylon fabric with a specific foam firmness. The sweeping movements themselves are also strictly regulated. Techniques that slow the stone down were prohibited in January 2026: it’s illegal to make single sweep pushes without a subsequent pull, for example.
Despite all the advances in technique and technology, though, the mystery remains: why do the stones curl in the way they do? Here physicists are uncertain, though plenty have claimed to find the answers.
The first effort to understand the stone’s curl was in 1924, when Canadian scientist E L Harrington at the University of Saskatchewan in Canada proposed the “left-right asymmetry theory”. In short, this blamed differences in friction between the left and right sides of the stone. If the stone is rotating, Harrington reasoned that one side spins in the direction of the stone’s travel; the other against, leading to slight differences in friction. However, it soon became clear that this theory was insufficient to explain everything going on.
How before why
Since then, various other models have been proposed to explain the odd physics of the curl – but none quite nailing it enough to achieve scientific consensus. To name a few, there’s the “water-layer” model, the “snowplow”, the “slip-stick” mechanism, the “scratch-guiding” process, and many others.
One of the most recent theories, in 2022, came from physicist Jiro Murata of Rikkyo University, Tokyo, who otherwise specialises in the physics of particles and higher dimensions.
Rather than start with mathematical modelling, Murata began by filming the curling stones precisely. “Most of the discussion about the origin of the curling stone motions has not been based on precise enough observations,” he says. “I believe that is the key reason we have spent a century trying to solve this mystery. Before we start thinking why the stone curls, we should observe how it curls.”
Via these observations, Murata noticed the stones seemed to rotate around a certain point, which led him to conclude that it was moving somewhat like a pendulum. “The spinning itself isn’t what pushes the stone sideways. Instead, the spin creates a difference in friction, and that friction acts as a pivot point,” he says. As a theory, it’s not a million miles away from Harrington’s 1924 theory, he acknowledges.
“If you grab a pole on your left while running, you will swing to the left around it. The curling stone behaves the same as that. If the stone’s rough bottom surface catches the ice on the left side, the stone curls to the left. The key takeaway is that the spinning itself isn’t what pushes the stone sideways. Instead, the spin creates a difference in friction, and that friction acts as a pivot point – just like the pole in your hand – that directs the stone’s path,” Murata explains.
In 2024, Murata also explored the effect of sweeping on the curl. Surveys of curlers suggest that there’s disagreement on how sweeping affects the bend of the stone. Among Japanese curling instructors, two-thirds recommended sweeping on the outside of the curve, while a third advocated the inside of the curve.
To settle the debate, Murata teamed up with two students who were also members of the university’s curling club Hinako Sonobe and Eri Ogiwara. Through a series of experiments, they confirmed that sweeping on the outside of the curl enhances the angle of the bend.
Why? Adding additional meltwater reduces friction there, so that the inside of the stone has relatively greater frictional contact with the ice, meaning it will swing more sharply around that point – like the pole-grab effect Murata described earlier.
Is this the final word on the physics of curling? Almost certainly not, and it wouldn’t be the first claim to have solved the mystery. Several other researchers have ideas of their own, so for now there’s no clear consensus. There are also many other variables to consider: the pebble condition, the chemical composition of the ice, temperature, humidity, microfractures, and more.
Like the debates over curling techniques among both amateurs and elites, no doubt efforts to explain the physics of this perplexing sport will continue to be hotly contested – or perhaps more accurately, coldly strategised.
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