From: Julie Yu (jhcyu@hotmail.com)
Date: Tue Nov 21 2000 - 23:27:09 PST
From: "Julie Yu" <jhcyu@hotmail.com> Subject: Re: frictionless ice Date: Tue, 21 Nov 2000 23:27:09 -0800 Message-ID: <F117t859I09Ek8fIpWI0000171e@hotmail.com>
Paul,
This "ice skating myth" has certainly been bothering me since I read about
it. I was convinced that the myth must really be true, and then today in my
thermodynamics class, my professor begins to talk about how the negative P/T
slope between liquid and solid water is the reason behind how ice skates
work. As if reading my mind, he then went on to talk about how the layer of
"water" on top of the ice is so small, that it is negligible when compared
to the blade of a skate. Which makes physical sense to me, after all, what
is 20 layers of molecules? He went on to say that the layer theory, not the
pressure theory, is the myth. You mentioned that the pressure under the
skate blade is too low by an order of magnitude to make any difference on
the ice. I know that I did a calculation on this in undergrad, so, torn
between two genuises I went ahead and ran the calculations myself. i may
have gone wrong somewhere, but assuming a 80 kg human and a blade edge of 20
cm x 2 mm, I got a pressure of ~1960 bar. This translates to a new melting
temperature of -14.5 C (using Clausius-Clapeyron and the triple point as a
reference). This is surely low enough to create a liquid layer under the
skate isn't it? i'm sure skates are probably a bit bigger, but so can be
humans.
also, as i think someone else mentioned, why don't other things slide on ice
if the sliding is inherent to the upper layer of water-like molecules?
simlilarly, why can't we skate on glass? by the layer reasoning, isn't the
layer of water molecules on glass sufficient to provide sliding?
Please shed some light for me! I apologize for the length.
julie
>On Sat, 11 Nov 2000, Paul Doherty wrote:
> >
> > Ice is frictionless because the top twenty layers of water molecules in
>solid ice behave like a liquid in their structure and properties. These
>molecules easily slide over each other. So even if the rubber of the tire
>sticks to the top layer of water molecules, these molecules slide over the
>ones below just as they would in a liquid.
> >
> > (This is the new theory of how ice skaters skate too, the old myth that
>the pressure of the skate blade melts the ice is now known to be wrong.
>Water ice will melt under pressure but the pressure at the skate blade is
>too low by more than an order of magnitude.)
> >
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