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BLACKBOARD I
The lriclion and pre$$tlre ol slralinU
Also glaciers, pressure cookers, and the Carnot theorem
by Alexey Chernoutsan
F YOU WERE TO ASK A TEN-
lear-old child why ice skates
glide so easily on the ice, the an-
I I*"r you'll probably get is the
simple and obvious one: "The skates
rub against the ice and they make a
thin watery film, and this helps the
skates slide along the ice." A student
more experienced in physics,
however, would find this answer too
simple and rather boring. "No, no,
no," the budding physicist will say,
"it's not a matte'r of friction but of
the pressure of the skates on the ice.
As the pressure increases, the melting
point of ice becomes less than
0oC, so the ice melts under the
skates." There is some merit in this
explanation-the melting point of
ice actually does decrease with an
increase in external pressure. However,
physics is a quantitative science.
If we want to find out whether
this physical phenomenon has any
bearing on ice skating, we need to
produce the appropriate numerical
calculations.
First of all, what is the melting
point of ice, and why is it interesting?
As you may know, when the
temperature increases to this point,
it can't be raised any further, and
any additional heat goes into melting
the ice. If no heat is provided
from outside, the ice and water coexist
in thermal equiiibrium. Thus, the
meltingpoint is the equilibrium temperature
of water and ice at a given
pressure. For example, it's equal to
0'C when the pressure is 1 atm.
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