Ice Skating - Physical Mechanics of Skating

Physical Mechanics of Skating

Ice skating works because the metal blade at the bottom of the skate shoe can glide with very little friction over the surface of the ice. However, slightly leaning the blade over and digging one of its edges into the ice ("rock over and bite") gives skaters the ability to increase friction and control their movement at will. The blade only moves forward and backward, and any violation of that principle results in skidding. In addition, by choosing to move along curved paths while leaning their bodies radially and flexing their knees, skaters can use gravity to control and increase their momentum. They can also create momentum by pushing the blade against the curved track which it cuts into the ice. Skillfully combining these two actions of leaning and pushing— a technique known as "drawing"— results in what looks like effortless and graceful curvilinear flow across the ice. How the low-friction surface develops is not known exactly, but a large body of knowledge does exist. These are explained below. Experiments show that ice has a minimum kinetic friction at −7 °C (19 °F), and many indoor skating rinks set their system to a similar temperature. The low amount of friction actually observed has been difficult for physicists to explain, especially at lower temperatures. On the surface of any body of ice at a temperature above about −20 °C (−4 °F), there is always a thin film of liquid water, ranging in thickness from only a few molecules to thousands of molecules. This is because an abrupt end to the crystalline structure is not the most entropically favorable possibility. The thickness of this liquid layer depends almost entirely on the temperature of the surface of the ice, with higher temperatures giving a thicker layer. However, skating is possible at temperatures much lower than −20 °C, at which temperature there is no naturally occurring film of liquid. When the blade of an ice skate passes over the ice, the ice undergoes two kinds of changes in its physical state and a change in temperature due to kinetic friction and the heat of melting.