Gliding for Gold: The Physics of Winter Sports

2. Skating on Thin Ice

2 SKATING ON THIN ICE We have seen how thin the ice is—one inch or less—for indoor ice sports such as speed skating and hockey. In this chapter we examine the physics of movement over ice, using ice skates. The act of skating is in many ways unnatural, and yet figure skaters make it look graceful, elegant, and artistic; speed skaters are the fastest people on two feet; and hockey players create a frenetic mayhem on ice in the fastest-paced team sport in the world. T H E S K AT E S If you are not a skater, you would be forgiven for thinking that an ice skate is simply a boot with a thin metal blade attached to the sole. In fact, ice skate design is much more complicated than that, as ice skates have evolved specific forms to match di√erent conditions and requirements—a testament to the long history of ice sports. Before describing these forms, there are a couple of general features that I need to tell you about. In figure 2.1a I have sketched an ice skate blade as seen from the side and from the front. From the side, you can see that the blade is not straight— the section that is in contact with the ice is curved upward at both ends. The blade is known as the rocker and its radius of curvature, which is di√erent for di√erent sports, as the rocker radius. This radius can change along the blade: for some types of skate it is less at the front of the blade than at the back. From the blade end you can see that the bottom surface is not flat, but instead is hollowed out. The depth of the hollow is described by the radius of hollow which, like the rocker radius, changes from sport to sport. The blade material is made of carbon steel, which is very hard. 30 GLIDING FOR GOLD

 
 Figure 2.1. Ice skate blade design. (a) When viewed from the side, the blade is curved, with a rocker radius that varies from sport to sport. Viewed from the front, the blade is hollowed out, to provide sharp edges for gripping the ice. The radius of hollow varies with the sport and the skater. (b) During the power phase of a skating stride, the hinged blade of a clap skate (right) stays in contact with the ice for longer than the rigid blade of a traditional skate (left). Despite being so hard, ice blades frequently require sharpening, to maintain the edges on either side of the radius of hollow.1 What is the purpose of these two types of curvature in a skate blade? A very curved rocker makes it easier for the skater to turn, and so sports that require great maneuverability, such as hockey or figure skating, are played on short-bladed skates with a short rocker radius. There is no free lunch: reduced stability is the price paid for this increased ease of turning. These characteristics make sense; there is greater forward and backward freedom of movement for blades with short rocker radius than for flatter blades with a long rocker radius, accounting for maneuverability with the former 1. An interesting article on the importance of skate sharpening can be found in New York Times (2009). SKATING ON THIN ICE 31 and stability with the latter. The radius of hollow gives blades sharp edges and so permits the skater to lean over without slipping, as when taking a bend. Consequently, sports which require the skater to make sharp turns, such as figure skating or hockey, require sharp-edged skates (with a small radius of hollow). Generally, the best radius of hollow for a given skater depends not only upon the sport but also upon the ice temperature and the skater’s weight and skill level—less skilled skaters are better o√ with large radius-of-hollow skates. Now that you know about the general features of ice skates, I can describe to you the di√erent skate designs that have evolved for di√erent ice sports. Figure skates (fig. 2.2) are very sturdy, with a blade that is quite thick, at 4 mm, and with strong ankle support. The rocker radius is small, at 2 m, and is even smaller near the front of the blade, to facilitate spins and turns. Figure...