Ice skating

Ice skating is traveling on ice with skates, narrow (and sometimes parabolic) blade-like devices moulded into special boots (or, more primitively, without boots, tied to regular footwear). It is mainly done for recreation and as a sport.

How it works

Ice skating works because the metal blade at the bottom of the ice skate boot 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 ("rockover and bite") gives skaters the ability to increase friction and control their movement at will. In addition, by choosing to move along curved paths whilst 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.

Related Topics:
Friction - Radially - Knees - Gravity - Momentum - Curvilinear

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In the past, the explanation for the low friction between the metal blade and the ice surface was that the pressure exerted by the blade on the ice caused the surface of the ice to melt. It was believed that this thin layer of water allows the blade to glide over it with very little friction. This hypothesis predicts that the greater the pressure exerted by the blade, the slipperier the surface of the ice will become.

Related Topics:
Pressure - Hypothesis

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However, it was found that the classic explanation was not the whole story. Experiments have shown that the pressure exerted by the blade has no effect on the amount of friction generated. The easiest proof of this is that a small child can skate on the ice as frictionlessly as a much heavier adult ice skater. The pressure required to melt the ice in this manner is about 10 times more than any ice skates generate, and the ice would shatter underneath the skates at that point.

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Further research in materials revealed the true nature of skating. Because the atomic structure of ice is a crystalline structure, it turns out that having this structure abruptly stop when it reaches the top of the ice is not the most entropically favorable form. Instead, there is always a thin film of liquid water ranging in thickness from only a few molecules to thousands of molecules on top of the ice. This allows a smoother transition from the structured ice to the completely random structure of the air molecules. The thickness of this liquid layer depends almost entirely on the temperature of the surface of the ice (higher temperatures give a thicker layer), and the liquid layer disappears around -20°C (-4°F). At temperatures below -20°C, ice skating becomes impossible because friction drastically increases and it feels like skating on glass. Experiments show that ice has a minimum of kinetic friction at -7°C, and many indoor skating rinks set their system to a similar temperature.

Related Topics:
Crystalline structure - Entropically favorable - Thin film - C - F

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