The modern game of curling consists of a 41 pound granite rock sliding across some 42 m of ice to a target called a house. The rock, technically termed a “stone,” is typically preceded on the ice by two players with “brooms,” who vigorously sweep the ice immediately in front of the stone to influence its trajectory (Willoughby et al. 2005, Bradley 2009, Esser 2011). This unique winter sport, affectionately termed the “roaring game” because of the sound the stone makes as it slides on the ice, has its roots in 16th century Scotland, where river bottom rocks were the stones which slid across ice-covered lochs to a target (Clark 2008). Since its revival in the mid-1800s in Scotland, curling has somewhat switched continents with Canada now being the world’s main curling powerhouse. Curling has grown to an Olympic sport and enjoys a unique niche of fans and followers (Holt 1989, Tranter 1989).
From the view of science, curling is a game about friction. The friction of the stone on the ice, the brooms on the ice rink, and even the footwear of the curlers on the ice, makes the game possible. Curlers wear a sliding shoe, which has a lower friction coefficient than the ice, and a non-sliding shoe, which has a friction coefficient larger than that of ice, giving the players the ability to step and slide (Dassler 1986). The sweeping of the brooms varies the friction coefficient of the ice, making it possible to influence the trajectory of the stone once it has already been pushed toward the house. A covering for the handle of the brooms has even been proposed, to lessen the friction (and thus wear) of the broom on the curlers’ clothing, a long recognized problem for curlers (Robertson 1987). The entire game centers on creating and decreasing friction to one’s advantage.
In order to understand the science behind the sport, a more thorough description of the physical characteristics of the rink and the stone is needed. First of all, the ice on the curling rink is not polished and smooth. Rather, it has a pebbled surface made by spraying the rink with water and allowing the tiny droplets to freeze on its surface. The pebbled surface is critical to the game. As for the stone, which is made of a very hydrophobic granite traditionally from northern Wales or the island of Ailsa Craig in Scotland, its under-surface is not flat either. Instead, it is slightly hollowed out, making room for a small air pocket and causing only a thin annulus around the bottom to make contact with the rink. Coupled with the fact that the ice rink is pebbled, the annulus actually only touches a few of the little pebble protrusions on the ice, making its true contact with the ice very low indeed (Shegelski 2001). The pebbled surface and the stone’s small hollow also do away with the potential problem of liquid getting trapped under the stone and suctioning it to the rink surface.
There has been much debate among scientists about how the different sources of friction influence the game. Of particular interest is the three-way interaction between the stone, the ice, and the sweeping of the brooms and how the interaction of all three causes the stone to “curl,” i.e. have a trajectory which curves, giving the sport its name. This mysterious curling motion has received a lot of attention, although a complete understanding of the motion is still lacking. To begin to understand the curling motion, one must first understand the physics of how the stone glides across the ice. One thought is that as the stone slides along the ice, the vigorous broom sweeping directly in front of the stone causes the ice temperature to raise just enough to create a thin lubrication layer of water from the melted ice, reducing the coefficient of friction that the stone is traveling over (Marmo et al. 2006). Another viewpoint, however, points out that there cannot possibly be a lubrication layer of water supporting the stone because the stone rests on a pebbled, not a flat, surface (Denny 2002). The lubrication layer would simply never accumulate because the liquid would spread out between the small pebbled ice protrusions that the stone rests on. On the other hand, since the stone rests on a pebbled surface, each individual protrusion receives more pressure from the stone, thus melting the ice on the surface of the protrusions and creating a lubricating layer even more easily (Shegelski et al. 1999). Regardless of the mechanism, it is clear that the friction coefficient of the ice does change with sweeping, and the pebbled surface also influences curling. Studies have shown that on a smooth ice surface, little to no curling occurs. On a dry surface, the friction coefficient is constant, but the effects of a stone on such a surface cause it to behave in the opposite way than on ice (Jensen et al. 2004).
So why does the stone actually curl? It has been suggested that it may be because of an asymmetrical buildup of friction. If, for example, a stone is sent forward with a slight clockwise twist, any debris on the ice will be pushed to the right side of the stone. Any liquid layer that forms from the pressure and friction will also push the water to that right side of the stone, slightly pooling the water on one side. This means that on the right and front side of the stone, there is more lubrication from the melted ice, so the stone as a whole experiences an asymmetric coefficient of friction (Bradley 2009). Additionally, if a sweeper stands to the side of the advancing stone to sweep, more heat is generated in the part of the sweeping stroke that is closest to the sweeper, so the ice will have a larger decrease in the friction coefficient on that one side of the stone (Marmo 2006). Additionally, if the stone moves too slowly, it may actually increase the coefficient of friction because as the heavy stone slides across the pebbled surface, its pressure and heat of movement causes the small ice pebbles to break off, leaving small pebbles to grate underneath the advancing stone (Denny 2002).
Despite these theories, one model concludes that no matter how the coefficient of friction is changed and distributed across the different sides of the stone, the real motion of a curling stone remains entirely unaccounted for compared to the models (Nyberg et al. 2012).
How one sweeps also affects the way the ice melts. Sweeping plays a large role in the game by simply raising the temperature of the ice so that the friction coefficient decreases and with the raised temperature, it will be all the easier to melt the ice once the stone hits where the brooms have just swept. As the ice is brought closer to its melting temperature, the energy it takes to completely melt it decreases (Marmo et al. 2006). Next comes a question as to whether one ought to sweep harder or faster, with a low angle with respect to the ice or with a high angle. And does sweeping play a larger role in altering the friction coefficient by polishing the ice, or is its chief role to melt the ice? Since the ice is orders of magnitude smoother than the stone, any polishing done to the ice has a negligible effect, although sweeping aside any debris is useful, and we see that the key use of sweeping is how it raises the temperature (Marmo 2006). We thus see that the key use of sweeping is how it raises the temperature of the ice.
Even the styles of sweeping also make a difference in the amount of friction present in the game – sweepers may use the conventional style, which consists of sweeping in front of the stone while standing to the side, or a high-angle style in which the sweeper standing behind the stone reaches over the stone to sweep in front of it. The high-angle style produces a stroke that overlaps each preceding stroke, giving a thermal history of ice at a more elevated temperature. The friction coefficient is thus lowered since each patch of ice is swept (and heated) several times. This is a clear advantage of this style – however, the drawback is that the high-angle style is farther away from the stone than the conventional style, meaning that although the ice experiences a greater increase in temperature when utilizing the high-angle style, the ice has a longer time to cool back to freezing before the stone reaches it, unlike the conventional style (Marmo 2006). There is also the question of whether it is better to sweep faster or harder. Sweeping twice as hard produces twice as much heat (Bradley 2009) which is desired in some instances, but in others, it is more advantageous to sweep faster so that the strokes can overlap each other and warm the ice more in that way. Regardless, when sweeping is done at the proper moment with the right technique, the power of friction can be leveraged to add 20-30 feet to the distance a stone can travel (MacDonald 1996).
The competing sources of friction and more particularly, the various means to alter friction coefficients during the game, give curling its unique qualities and intriguing scientific questions. As curling continues to gain popularity, it can be expected that further scientific inquiry will continue to be directed at exploring the role of friction in the curl of the stone, until it can be answered with certainty how the asymmetric friction coefficients acting on the stone cause it to curl.
By: Jessica Egan, University of Utah
Jessica Egan began her love of chemistry under her mom’s direction with homeschool experiments in middle school and then up into high school through her high school chemistry teacher. She pursued a chemistry and art double major at Hillsdale College and decided to continue her chemical education by attending graduate school at the U. She has recently graduated with a M.S. from the U in Analytical Chemistry and is looking for a career in industry.
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