Or, why you can
glide on
skates.
Water has a very novel
phase diagram in that increased pressure on the
solid form of
water (
ice) can force the substance to
melt. (In many substances, the application of
pressure would lead to
solidification or
freezing.)
Ice skates concentrate the
weight of a
skater onto a very small
area of
metal in
contact with the
ice surface. Since
pressure is the
force (here, the
weight of the
skater) per
unit area, the result is relatively
high pressure immediately underneath the
skate's
blade. A thin
layer of melted ice (
water) forms, and this is what allows a
skate to
glide on
ice. Note that the
temperature of the ice/water has not changed. The effect is
not due to
friction. (If there were
friction, you wouldn't
glide.)
I now realise that this refers to why does ice float?. But i like this write-up, too.