brewnog said:
http://www.physlink.com/Education/AskExperts/ae357.cfm
Yegor said:
I don't find the calculation in the first article very satisfying. The assumption that a blade is about 1/2 cm wide for calculating the pressure is way off base. Skate blades are ground with a hollow between the edges to limit the contact surface to just the edges of the blade. It is not the width of the blade times the length. The conclusion that the calculated pressure is only sufficient to raise the temperature a fraction of a degree is unsupported in the article, but I do think it is correct for the calculated pressure. It appears to be based on the phase diagram for water which shows that it takes an enormous amount of pressure to change the melting temperature slightly. I seem to recall this slope is found from the Clausius-Clapyron relation involving the latent heat and change in volume at the phase transition. For water it is about -1.23 x10^7 Pascals per degree C. But there is no account taken in this article about the motion of the blade relative to the ice. We all know that if your hands are cold and you want to warm them up you don't just press them together. You rub them to generate heat. There is no consideration of heat generated by the relative motion of the blade and the ice in this analysis.
The second article describes an investigation into the nature of the surface of the solid, which for any solid has to be different from the bulk material. You cannot expect the bulk properties, where every mollecule is completely surrounded by like molecules, to be the same as the surface properties. Which raises the question of how can you expect to refute the idea that ice skates melt ice at the surface by doing a simple pressure calculation that applies to the bulk material.
I think both articles make the point that there is something about the surface of ice that is different from the bulk properties that is somehow connected to reducing the friction of ice skates, but neither of these articles refutes the idea that the ice under the blade is melting. Perhaps the conclusion to be drawn from the second article is that it is a lot easier to melt a little bit of ice at the surface by applying pressure to it than one might predict by considering the application of pressure to the bulk material.
The question of whether the ice melts under the skate would best be answered by looking for water in the skate trail immediately after the skate has moved on, or for a change in the surface . Why is it that on fresh ice you can always see a trail of the skate? It is because the ice that was there is not where it used to be, even if the skate is moving perfectly straight. The ice has not been scraped away as it is when the skates move sideways. Where did it go? If you get an ice cube out of your freezer and run a knife over it, the knife leaves a depression, even if the pressure is very small. It leaves a depression because the ice melted under the blade. You can see it and you can feel it. The ice that was there is gone.
The point is, neither of these articles debunks the "myth" that friction is reduced by a layer of slippery water under the skate blade. What may well be true is that you cannot account for that melting based on a simple pressure calculation relating to the bulk properties of ice, but you might very well account for it with a better understanding of the nature of the ice surface.
