There is still some debate over this, but the
current view is that ice is slippery because of
a layer of water molecules present at the
surface. These water molecule roll between the
ice and the object (here, an ice skate) pressing
against the ice, like tiny ball bearings.
[For more advanced readers, a recent journal
The force required to "roll" these water
molecules is much lower than that to slide of the
skate against a dry surface. This liquid layer
decreases in thickness with temperature, becoming
practically nonexistent at ~-100°C (212°F).
This means that ice below that temperature is
not slippery. In addition to this
"liquid-like" layer, there is probably an
additional contribution from friction between the
skate and the ice. Although the friction
between the skate and the ice is low, it is not
nil. As a result, sliding against and
deforming the ice produces some heat which
may melt a small amount of ice and contribute to
the lubricating layer.
One hypothesis for the low friction which has been
quite conclusively disproved is that the top layer
of the ice melts because of the pressure of the
skate blade. The explanation for the hypothesis
comes from an examination of the
of water. [A phase diagram is basically a plot
showing combinations of environmental conditions
where different phases of a substance, e.g. liquid
water, ice, and water vapor, are able to exist.]
Water is one of a small number of substances which
has a negative slope to the boundary
between the solid and liquid phases. This means
that ice can be made to melt at a temperature
below 0°C (32°F) by increasing the force pushing
down on it. This force might come, for example,
from an ice skater.
While this will work in theory, the temperatures
of ice rinks is low enough that the pressure
increase required to melt the ice is far greater
than that produced by any ice skater. Thus, this
cannot be the explanation.
is an article which discusses the competing
theories. It is beyond a first grader, but is not
so scientific as the journal article.]
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