> From: Cliff Swartz > Date: 30-OCT-1996 > To: Ludwik K. > Hello, Ludwik > Let me expand the information about ice skating by citing the > section on the topic from my new book "Teaching Introductory Physics", > published by the American Institute of Physics.(p 93) > "Ice skates experience low friction because a thin water film is > produced between blade and ice. A popular legend has it that this > effect is caused by the lowering of the melting point due to the pressure > exerted by the blade. It is true that one of the unusual features of > water is that it expands during freezing. Consequently, if we prevent > the expansion by increasing the pressure, we prevent the freezing and > thus lowwer the melting point. The usual pressure-temperature diagram > for water, Fig 4-5, shows the boundary line between solid and liquid > arching back to the left from the triple point. However, the actual > effect is very small. > The slope of the boundary line is -(1.2x10^7 N/m^2/C) It would take an > increase of 120 atmospheres to lower the melting temperature 1 Celsius > degree. For the typical skate blade, the area is 27 cm x 4 mm = 11 cm > If the full weight of a skater with mass 65 kg is exerted on one blade, > the increased pressure would be about 6 atm. Sharpening the blades > does not decrease the contact area appreciably, since the blade sinks > down into the relatively soft ice. > What does produce the water film between blade and ice? There are two > plausible explanations. When the leading edge of a blade strikes the > ice, the resulting friction energy can melt a trail for the rest of the > blade. A more important effect stems from a phenomenon first noticed > by Faraday and then largely ignored because it was not understood. > We now know that at the interface between ice and air there is a fhin > film of water. The thickness increases from monomolecular to several > hundred molecules as the temperature rises from -10 to 0 C. Since the > reduction of friction depends on the the water film, you might conclude > that the fastest skating could be done on ice close to the melting point. > However, warm ice is soft ice, allowing the blades to sink in more. On > the other hand, cold ice, which is hard, has only a thin film of surface > water. These two competing effects yield a minimum of friction for > speed skating at about -7 C. (For further details. see James White, The > Physics Teacher, 30, 495 ((1992))." > Also note that any explanation must account for the very low friction > experienced by the puck. I hope this explanation will either satisfy > people or stir up further controversy!