Skating on thin water.

There's nothing really simple about skating smoothly across a sheet of ice.

One standard explanation of why skates slide so easily on ice holds that the pressure produced by a skate's sharp blade forces a little of the ice to melt, creating a thin, slippery film of water on which the skate actually glides. But this answer doesn't hold up under close scrutiny. Samuel C. Colbeck of the U.S. Army's Cold Regions Research and Engineering Laboratory in Hanover, N.H., reviews some of the arguments against pressure melting as the cause of the low friction encountered in ice skating and snow skiing in the October American Journal of Physics.

Colbeck argues that the pressure needed to reach the melting temperature of ice would more likely cause the ice to crack and fragment. Even if melting did occur, only an exceedingly thin film of water would be present. "Pure liquid water cannot coexist with ice much below -20oC at any pressure," he adds, "and friction does not increase suddenly in that range." Skating and skiing are still possible below this temperature.

Heating caused by the friction of a skate moving rapidly across the ice represents an alternative mechanism for the formation of a water film to facilitate skating. "This mechanism generates heat at the interface where it is needed, by the shear of the thin water film," Colbeck notes.