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Wikipedia on-line encyclopedia - skis & skiing

post #1 of 6
Thread Starter 
Over on Snowheads, Comprex brought to people's attention the entries in Wikipedia about Skiing. I also rather enjoyed their entry for "Parallel Turn":

http://en.wikipedia.org/wiki/Ski and

Here's what I wrote in response:

In the 2nd paragraph, I kinda liked the line, "... Ski wax is used to increase the freezing point of water ..."

I then clicked on the link to "skiing", followed some links and found their description of a parallel turn:

"... The technique involves setting the uphill ski so it runs along its big-toe edge, and applying pressure to it. Because of the way skis are shaped (narrower under the foot) the ski will turn (to the left if the pressure is to the right ski, to the right if the pressure is on the left). As the turn progresses the two skis remain parallel, and the second ski also starts to run along the inner edges.

The turn can only be made if travelling at some speed, since the speed of the skier generates the pressure needed to turn the skis properly as the skier banks into the turn. ..."

I think that this should definitely be brought to the attention of Bob Barnes, who, as we know, wrote the REAL encyclopedia of skiing.

Tom / PM
post #2 of 6
I've also heard that the whole idea of skis and ice skates making the snow/ice melt is bunk. I heard a long, scientific explanation of it which I didn't understand. Oh well, someone else may enlighten us.
post #3 of 6
Thread Starter 
Actually, the idea of friction causing the snow to melt when under the pressure of a moving ski is true.

What is not true is that wax causes the freezing point to rise. Even if this were true, it wouldn't be very useful as it would have water turning solid at higher temperatures, say, 45 deg F, ie, more easily, not retard the freezing process.

They probably meant to say the opposite, that wax causes the freezing point to fall, but this isn't true either. Wax does nothing to the freezing point - it varies the adhesion of water to the base at higher temps, and varies the degree to which hard, sharp snow crystals can stick into microscopic irregularities and tortuosities in the base in very cold temps.

Tom / PM
post #4 of 6
No, they were very clear that they were debunking the myth of the melting snow/ice. It was a TV show on PBS, but I don't remember which one. Don't know any more but I'm sure of those things.
post #5 of 6
Thread Starter 
You are probably referring to a show that was prompted by a report by Professor Gabor Somorjai of the Lawrence Berkeley National Laboratory, eg:


The relevant part says:

"... In the past, scientists believed that either pressure or friction melted the ice, creating a water lubricant that allows skates and pucks to slide. Berkeley chemist Michel van Hove, a collegue of Somorjai's, has done calulations which show that skates and pucks do not generate enough pressure to instantly liquefy ice. Somorjai has discovered that ice has a "quasi-fluid layer" that coats the surface of ice and makes it slippery. ..."

I have served on an NSF committee with Somorjai. What he said is true. However the calculations that his colleague, van Hove, did are in error.

Yes, Somorjai is absolutely correct that there is a disorganized, quasi-liquid-like layer of water molecules several atoms thick on most surfaces, and yes, this contributes to the relatively low coefficient of friction of ice even when there is no pressure between the two opposing surfaces. These are results that came out of imaging studies using the relatively new atomic force microscope technology developed in the 90's (and other studies).

However, when there is pressure between the two surfaces (ie, the snow and the base of the ski), the thickness of this water layer increases dramatically - from a few atoms thick to many microns thick.

Contrary to the calculations of van Hove, this increase in the thickness of the melt layer has been observed experimentally. People have taken actual pictures of it. If anyone thinks that high temperatures can not be achieved under a ski, has not seen edge burn on a racing ski.

The difference that this increase in the thickness of the melt layer causes in friction is the same as what you experience if you have a thick layer of water between two sheets of glass, versus having a very thin layer of water between two sheets of glass. In the latter case, it is almost impossible to move one piece of glass with respect to the other, whereas in the former case it is easy to slide them with respect to each other.

Although the book, "The Physics of Skiing"

came out before the AFM results, it contains a very good explanation of the increase in thickness of the meltwater layer, and a comparison of the this source of friction on skis to other sources of friction such as snow compaction, snow displacement, dry snow friction, wind resistance, etc.


Tom / PM
post #6 of 6
Looks like there's no longer any hurry to get my next edition out, with this first-rate resource on the 'net!

I like this one:

Uphill ski refers in this case to the back end of the ski, i.e. behind the skier.

Downhill ski refers to the front end of the ski. i.e. in front of the skier.
(from this page about stem christies.)

But the rest of the description of stem christies seems to go back to the "normal" use of the terms.

Best regards,
Bob Barnes
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