Australia’s first ever Olympic curling team made a historic win but missed the medal podium at the 2022 Winter Olympics in Beijing. It was a remarkable achievement for a team lacks any dedicated curling facilities home.
And this is important, because it is the special properties of curling ice that allow the heavy curling stones to slide and bend in ways that seem to defy the physique. In fact, researchers are still unsure of what puts “curl” in curling.
Chess on ice
Curling’s origins go back to 16th century Scotland, which it does one of the world’s oldest team sports. Just like golf – invented at about the same time in the same part of the world – curling seems both entertainingly pointless and deceptively simple to the untrained eye.
It has been called “chess on ice”, although for many Australians it most resembles frozen grass bowls. Athletes take turns sliding circular 20-pound granite stones along the ice toward the center of a horizontal target 28 feet away. Teams are awarded points for getting their stones closest to the center of the target, or “house”.
Read more: Why curling is so gripping to watch
The slippery science behind curling begins with the ice itself. Curling ice must be perfectly flat – much flatter than a regular ice hockey rink – and sprayed with water drops before each match to create a rocky surface. This minimizes the contact area between the ice and the heavy curling stone.
Curling stones also have a concave lower surface – like the bottom of a beer bottle – which further reduces the contact area between the stone and the ice. The effect is to increase the pressure at the base of the stone, partially melt the ice and reduce the friction in a similar way as how skates work.
Unique among Olympic sports, curlers can change the path of the stone after it has been “thrown”. This is achieved by heavily sweeping the ice in front of the stone with special brooms that heat the ice and reduce friction, which allows the stone to travel longer and straighter along its path.
Deciding when, where and how hard to sweep has a big impact on the path of the stone; so of course it is accompanied by a lot of enthusiastic screams.
Give it a spin
By adding a small amount of spin, skilled players can make their stone “curl” along a curved path to block an opponent’s stone or knock it out of the way. Even a small amount of rotation can divert the curling stone’s path by as much as one and a half meters. How exactly the curling stone does this something of a puzzle.
Let’s start with a (literal) experiment for table tops. Slide an inverted glass along a table and add a small swirl as it leaves your hand. With a little practice (and maybe a few replacement glasses) you will be able to make the glass trace a curved path across the table, deflecting to the left when you spin it clockwise or to the right when you spin it counterclockwise.
The reason for this is explained by a branch of science called tribologywhich studies the effect of friction on moving and sliding objects.
When the glass spins, it rubs against the table top, creating friction that tries to slow down the glass’s rotation. The frictional forces are directed opposite the direction of movement: for a clockwise rotating glass, the friction will be directed to the left on the front of the glass and to the right on the back of the glass.
When the spinning glass slides over the table, it leans slightly forward in the direction of travel and presses the glass’s front lip a little harder against the table than the back lip. The extra pressure generates extra friction at the front compared to the back. The resulting imbalance of frictional forces causes the glass to deflect in the direction of stronger friction – to the left in the case of a clockwise rotating glass.
A twist in the saga
But curling stones behave in exactly the opposite way: a clockwise rotation causes the stone to bend to the right, not to the left. For a long time, researchers assumed that this was due to an effect called asymmetric friction.
The theory goes like this: like a glass sliding over a table, a curling stone leans slightly forward. The extra pressure on the front of the stone partially melts the ice in the front edge, which creates a thin film of water that reduces the friction at the front of the stone compared to the back.
The curling stone will still be deflected in the direction of stronger friction. But in this case, it is the trailing edge that wins, resulting in a deflection to the right rather than to the left, for a clockwise rotating stone.
Like many theories, this explanation was widely accepted until someone got started to actually test it. 2012 a team at Uppsala University in Sweden made detailed calculations of the frictional forces acting on a sliding stone.
The problem they found is that curling stones rotate quite slowly, only finishing a few laps before stopping. This spin is far too small to cause a lateral deflection of one meter or more. Even odd, more rotation does not lead to more curl – in fact, spin a stone too hard and it will not attract at all. Asymmetric friction can not explain such behavior.
The researchers used an electron microscope to take a closer look at the ice under a curling stone. They discovered that the leading edge of the stone leaves small scratches on the ice in the direction of rotation. These scratches act as a guide for the back edge of the stone, which causes the stone to bend in the direction of rotation.
The Swedish team then showed that with the help of this “scratch-guide” mechanism they could “steer” the sliding stones by adding artificial scratches on the ice in different directions. In one experiment, a stone was made to travel along a zigzag path by laying down scratches in alternating directions.
Their findings ignited one less controversy in the admittedly niche world of curling physics.
Competitive theories have been proposed, including pivot-slide modelthe evaporation-wear modeland that snow plow model.
In 2020, a Japanese team tried to sort things out systematically test each theory in a curling hall with the help of sophisticated motion tracking equipment, a laser microscope and a few sheets of sandpaper to modify the surface of the curling stone.
However, no clear winner emerged. When it comes to the science of curling, it seems like we’re just scratching the surface.
Author: Shane Keating – Senior Lecturer in Mathematics and Oceanography, UNSW Sydney