Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site cubsvax.UUCP Path: utzoo!linus!decvax!harpo!floyd!cmcl2!rocky2!cubsvax!peters From: peters@cubsvax.UUCP Newsgroups: net.physics,net.followup Subject: Re: More on Cold Bottles of Coke Message-ID: <136@cubsvax.UUCP> Date: Thu, 5-Jan-84 15:27:03 EST Article-I.D.: cubsvax.136 Posted: Thu Jan 5 15:27:03 1984 Date-Received: Sat, 7-Jan-84 03:17:05 EST References: <134@cubsvax.UUCP>, <631@houxz.UUCP> Organization: Columbia Univ Biology, New York City Lines: 50 Whoops! I just replied to my mail copy of a followup to my comments about skaters and the Clapeyron equation; now I see it's also been posted, so I'll try to reconstruct my mailed reply, which I didn't save. Basically, on a cold day, the temperature of the ice surface is the same as that of the air, though it may be colder deep below, at the ice/water interface. Ice is a poor conductor of heat. So, since we all know that it's possible to skate at 0 deg F, we'd have to achieve enough pressure to melt the ice at that temp, if the pressure-melting hypothesis is correct. But even if we could only skate down to 11 deg F, we'd need to produce 1500 atm (about 22,000 psi) of pressure. Someone pointed out to me (by mail) that skates are not only hollow-ground, but also convex, so let's consider a contact *length* of 3 in; this would require each blade to make contact over a width of about .0023 in for our 300 lb skater. I don't believe it. Blades cut into the ice, as one can see since they leave tracks, and this increases the surface area of contact, reducing the pressure. Furthermore, it's easy to slide on ice in one's shoes -- and you just *can't* say that you're melting the ice by pressure there. In reconsideration, I don't think frictional heating is the answer either. It's just probably that ice is slippery! Sharp skates work well because they enable one to direct ones momentum parallel to the blade, instead of skidding all over the place. Note that I never said that skating is impossible -- only that pressure- melting of the ice is unlikely in the process. I believe that even more strongly now, because of what I mentioned in the last paragraph. Incidentally, someone from a Bell Labs site sent me mail about the pressure in a Coke bottle; my mail reply was returned because of a routing glitch, so let me say here that I agree (as a careful reading of my first posting on this subject will reveal) that the pressure in a coke bottle is "more like 150 than 750 psi." But that confirms my notion -- also proposed by the individual who first brought up the Coke bottle on a cold day phenomenon -- that, in fact, supercooling, not pressure- induced freezing-point depression, was responsible. Also to this individual: the problems of sealing a pressure vessel are very different from those encountered when sealing a vacuum vessel. With high pressures -- say above 1000 atm -- one can use "unsupported area seals," which use the pressurized fluid to make the seal better, so that the higher the pressure, the better the seal. In such equipment, leakage is encountered at the lower pressures -- a few hundred bars -- only, unless a seal is damaged. See Bridgman's book on high pressure (Dover). Coke bottles may be closer to the way vacuum vessels are sealed. {philabs,cmcl2!rocky2}!cubsvax!peters (Peter S Shenkin; Dept of Biol Sci; Columbia Univ; NY, NY 10027; 212-280-5517)