For a sport that dates back to the 16th century, it’s surprising to know that we still don’t fully understand how curling works.
The winter Olympic sport — first practiced on frozen lakes in Scotland centuries ago — involves throwing granite boulders onto an icy, uneven surface so that they slide and curve toward a target known as the house. Players use brooms against the ice in front of each stone to control its trajectory.
It all seems simple enough, but as curling coaches themselves acknowledge, there are still many unknowns about the physics of the sport. Thus, although the basic tactics are consensual, there is often disagreement and controversy about the most effective techniques, even at a high level.
Of all the scientific mysteries, one of the biggest refers to the name of the sport itself: how and why do stones curve (“curl”, in English)?
If a player turns a stone clockwise when throwing, it ends its trajectory by curving to the right, and vice versa. With a basic understanding of physics, this is not what one would expect.
You can do an experiment yourself and observe. If you throw a glass face up onto a rug, it will curve in the opposite direction of rotation. Why don’t stones do this?
“The scientific community has not yet reached a consensus on the physics of curling, although not for a lack of effort,” says Jennifer Vail, author of Friction: A Biographyspecialist in “tribology” — which is the scientific study of friction, lubrication and wear.
“It has been more than a hundred years since researchers began trying to understand the phenomenon, but the mechanisms behind the stone’s curvature remain a mystery.”
Complex mechanics
While it may seem simple, curling involves complex mechanics — so it’s worth explaining why before addressing curling’s central mystery.
To begin with, granite stones, despite their appearance, are not ordinary blocks of rock. Taken from just two locations in Wales and Scotland, they are particularly resistant and waterproof. And the shape is fundamental: the bottom is concave, with edges called “rolling strips”, similar to the base of a beer bottle. It is this strip that comes into contact with the ice.
The ice is also specially designed. Unlike a typical ice rink, the surface is “grained” before starts, with tiny droplets of water spread over the top to create a rough surface.
“Without this grain, friction would prevent the stone from reaching the house,” says Vail. “This may seem counterintuitive, as potholed roads make us slow down when driving, but in curling, these potholes reduce the contact between the rock and the ice, which reduces friction.”
Additionally, there is 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 reduces friction, helping to keep the stone moving and influencing its trajectory.”
A 2024 research explored the three phases of the stone’s journey over the ice.
After launch, at its maximum speed, the greater volume of meltwater induced by friction causes it to effectively “aquaplane” in a straight line. Sweepers increase this distance by rubbing their brooms on the surface of the ice in front of the stone, creating extra lubrication with water.
As the stone slows down, the amount of water decreases and the abrasion of the hard ice begins to act. It is at this point that the stone begins to curve. Finally, when it stops, the water completely disappears and the stone experiences completely dry friction, stopping completely.
Players use different techniques to try to control the curve the stone makes, such as sweeping to one side as it slides or sweeping directionally at an angle, but scientifically it is often unclear why some techniques work better than others.
As broom technology has advanced, sweeping methods have also evolved. Sometimes this leads to rule changes.
In 2015, new brush materials capable of scratching ice appeared to give some players an unfair advantage. There was a technological doping scandal known as “broomgate”, leading the World Curling Federation (WCF) to ban certain types of brushes in 2016, allowing only smooth nylon fabric with a specific foam firmness.
The sweeping movements themselves are also strictly regulated. Techniques that slow the stone were banned in January 2026: it is illegal to make a single sweeping thrust without a subsequent pull, for example.
Despite all the advances in technique and technology, the mystery remains: why do stones curve the way they do? Physicists still aren’t sure, although many claim to have found the answers.
The first attempt to understand the curvature of the stone occurred in 1924, when Canadian scientist EL Harrington, from the University of Saskatchewan, in Canada, proposed the “theory of left-right asymmetry”.
Briefly, this theory attributed the phenomenon to differences in friction between the left and right sides of the stone. If the stone is rotating, Harrington reasoned that one side rotates in the direction of the stone’s movement, while the other rotates in the opposite direction, resulting in small differences in friction. However, it soon became clear that this theory was insufficient to explain everything that happens in movement.
‘How’ not ‘why’
Since then, several other models have been proposed to explain the peculiar physics of the stone’s curvature — but none have managed to explain it with enough precision to reach scientific consensus. To name a few, there is the “water-layer” model, the “snowplow” model, the “slip-stick” mechanism, the “scratch-guiding” process and many others.
One of the most recent theories, from 2022, came from physicist Jiro Murata of Rikkyo University in Tokyo, who specializes in particle physics and higher dimensions.
Instead of starting with mathematical modeling, Murata started by precisely filming the curling stones.
“Most of the discussion about the origin of curling stone movements has not been based on precise enough observations,” he says. “I believe this is the main reason why we spent a century trying to solve this mystery. Before we start thinking about why the stone curves, we should look at how it curves.”
Through these observations, Murata noticed that the stones appeared to rotate around a certain point, which led him to conclude that their movement was somewhat similar to that of a pendulum.
“The rotation itself is not what pushes the stone sideways. Instead, the rotation creates a difference in friction, and that friction acts as a pivot point,” he says. This theory is not far removed from Harrington’s 1924 theory, he acknowledges.
“If you hold a pole to your left as you run, you will make a left turn around it. The curling stone behaves the same way,” explains Murata.
“If the rough surface at the bottom of the rock grabs the ice on the left side, the rock curves to the left. The key takeaway is that the rotation itself is not what pushes the rock sideways. Instead, the rotation creates a difference in friction, and that friction acts as a pivot point — just like the post in your hand — that directs the rock’s trajectory.”
In 2024, Murata also explored the sweep effect on the curve. Surveys of curlers suggest there is disagreement about how sweeping affects the curve the stone makes. Among Japanese curling instructors, two-thirds recommended sweeping the stone on the outside of the curl, while one-third advocated the inside.
To resolve 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 the stone on the outside of the curve increases the angle of the curve. Why? The addition of melt water reduces friction in this region, causing the inner part of the stone to have relatively greater frictional contact with the ice, which means that it makes a sharper curve at that point — like the effect of picking up the pole previously described by Murata.
Is this the final word on the physics of curling? Almost certainly not, and it wouldn’t be the first time the mystery has been claimed to have been solved. Several other researchers have their own ideas, so for now there is no clear consensus. There are also many other variables to consider: the condition of the pebbles, the chemical composition of the ice, temperature, humidity, microfractures, and more.
Like the debates over curling techniques between amateurs and professionals, efforts to explain the physics of this intriguing sport will undoubtedly continue to be the subject of intense disputes — or perhaps, more accurately, cold and calculated strategies.
This text was originally published here.
