minipost
One of the questions on my Ph.D. physics prelim was this:
what is a ruby and why is it red? why is the friction so low on a sled? on a plot of p verses t, plot out the phases of helium-3
We had twenty minutes to impress the readers with whatever depth we could must. The middle question was a trick to see how foolish we were.
Many smooth surfaces aren't slippery. Ice can be slippery even if it isn't very smooth. The normal explanations are the weight of the skater puts pressure on the ice lowering it's melting point and/or the friction of motion heats the surface melting a thin layer. The problem is even if you use a heavy skater - say 120kg on small skates - you only get a liquid surface down to about -3.5°C. The friction is only good for a couple of degrees for the fastest speed skaters. I've (stupidly) skated in -30°C weather. What's up?
A century and a half ago Faraday hytophesized ice forms a thin liquid surface. His conjectures as to why and his experiments weren't good enough. Now it is known ice has a very thin layer of liquid water - about four or five molecules thick near freezing and down to two at -35°C where the skating begins to get tough. The pressure and friction effects are real and contribute, but only on warm ice.
It took an exotic experimental technique to verify and measure the effect and physics behind the pheomea is on the deep side, but we have something. And that's why, when you stick two ice cubes together and put them back in the freezer they freeze into a single body.
Those ice dancers, hockey players, speed and figure skaters - they're all gliding on a film four atoms thick.
How cool is that?
(speed skating often warms the air above the ice, but that's for the same reason they do it for indoor cycling - wind resistance)
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