rb@houxn.UUCP (R.BOTWIN) (01/06/84)
Why is the notion that the pressure of an Ice skater melts the ice false??
I thought that H2O was densest at 4 deg C and since it exists as a liquid
at that temperature, the pressure causes melting.
That was also the explanation for freezing pipes bursting, since unlike
other substances, water EXPANDS when freezing (below 4 C)
Rob Botwin
.....{utah-cs|seismo|decvax}!harpo!eagle!hogpc!houxn!rb
ATT/IS Labs (201) 577-5016 (Cornet 8-270-5016)
FJ 1B-130rpw3@fortune.UUCP (01/09/84)
#R:houxn:-44500:fortune:3500007:000:1855 fortune!rpw3 Jan 8 21:48:00 1984 Rob Botwin is exactly right. Ice skates ride on WATER! That's the reason those blades have SHARP! edges on both sides and a concavity in the middle: |///steel////| |////////////| | __------__ | |/ \| | | ^ ^ pressure points The system fails to work at two extremes: when it is too warm, the skater falls through the ice; when it is too cold (or the blades not sharp enough), the pressure (weight of skater/area of blade) is not sufficient to liquify the ice under the blade. The concavity allows a certain amount of self-adjusting of the blade area. In fact, with reasonably well-tuned skates, the pressure will be sufficient at temperatures well below where humans comfortably skate. For proof, look at what is called the "phase diagram for water" in any college chemistry book (in the thermodynamics chapter). Calculate the pressure under an ice-skate blade (say >100 lbs of skater divided by a 3 inch length (curved blade) by 1/25 inch width (dull blade) area ==> >800 lb/sq-in. See if your if your copy of the phase diagram doesn't predict liquid at that pressure at any reasonable skating temperature (say -20 F). In fact, given the shape of the blade, the weight of the skater, and the temperature one can predict exactly how far the blade will "cut" into the ice. (Ans: just far enough so that the area in contact with the ice divided into the skater's weight supplies just enough pressure to liquify the ice.) Don't let the fact that you never see liquid water fool you. The system is reasonably isothermal, so the water immediately re-freezes when the pressure is removed (the skater passes on). Rob Warnock UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3 DDD: (415)595-8444 USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065