>  From:   Cliff Swartz <CSWARTZ@ccmail.sunysb.edu>
>  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!