Re: Tom Kratman at Baen -- cringeworthy?

Ben Goodman <goodben@xxxxxxxxx> wrote in
news:71b77df5-d3a1-41ca-b862-84a5b3741807@xxxxxxxxxxxxxxxxxxxxxxxxx
com:
...
Slightly related to this, water is also very unusual in its
phase diagram in that it has a negative slope to its melting
curve. Applying pressure to ice causes it to melt (if the
temperature is kept the same) which is what makes ice skating
possible.

Maybe not. From a New York Times article a couple years ago:

***
According to the frequently cited ? if incorrect ? explanation of
why ice is slippery under an ice skate, the pressure exerted along
the blade lowers the melting temperature of the top layer of ice,
the ice melts and the blade glides on a thin layer of water that
refreezes to ice as soon as the blade passes.

"People will still say that when you ask them," Dr. Rosenberg said.
"Textbooks are full of it."

But the explanation fails, he said, because the pressure-melting
effect is small. A 150-pound person standing on ice wearing a pair
of ice skates exerts a pressure of only 50 pounds per square inch on
the ice. (A typical blade edge, which is not razor sharp, is about
one-eighth of an inch wide and about 12 inches long, yielding a
surface area of 1.5 square inches each or 3 square inches for two
blades.) That amount of pressure lowers the melting temperature only
a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters
can easily slip and fall at temperatures much colder.

The pressure-melting explanation also fails to explain why someone
wearing flat-bottom shoes, with a much greater surface area that
exerts even less pressure on the ice, can also slip on ice.

Two alternative explanations have arisen to take the pressure
argument's place. One, now more widely accepted, invokes friction:
the rubbing of a skate blade or a shoe bottom over ice, according to
this view, heats the ice and melts it, creating a slippery layer.

The other, which emerged a decade ago, rests on the idea that
perhaps the surface of ice is simply slippery. This argument holds
that water molecules at the ice surface vibrate more, because there
are no molecules above them to help hold them in place, and they
thus remain an unfrozen liquid even at temperatures far below
freezing.

Scientists continue to debate whether friction or the liquid layer
plays the more important role. Dr. Rosenberg, asked his opinion,
chose a indecisive answer: "I say there are two major reasons."
***
<http://www.nytimes.com/2006/02/21/science/21ice.html>

Mike

--
Michael S. Schiffer, LHN, FCS
mschiffe@xxxxxxxxxxxxxxxxx
.

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