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214    world’s fastest sled
skier who falls over in loose snow that is a
couple of metres deep can find himself in real
trouble. Because loose snow has so much air in
it, it compacts when the skier tries to put his
hand down to push himself up, and his hand
sinks in. Before he fell over he was far better
off because his skis, as wide as his feet but far
longer, spread his weight over a much bigger
area, so the snow could support him without
compacting as much.
Now we were starting to get to the bottom
of the question about why the expensive
toboggan didn’t go as well in deep snow as a
little lad sitting on a bin-liner. The toboggan’s
runners cut into the snow, and resistance from
the surrounding snow on the structure of the
toboggan and any parts of the rider that were in
contact with it meant that it was stuck there. On
the bin-liner, however, the rider was spreading
his weight more, so he didn’t sink in so far and
gravity was still able to drag him down the slope.
On an icy slope, however, things would be
different. The lad on the bin-bag would be off
like a shot, riding on that thin film of water
under his bum – but he would have no control
whatsoever. Any slight bump or shifting of his
weight would send him shooting off at an odd
angle. The toboggan’s runners would cut into
the ice, providing friction along their length
Top: Water molecules
have lots of energy and
tumble about, constantly
forming and breaking
bonds with each other.
Middle: At 0º Celsius the
water molecules have less
energy and form permanent
bonds, creating ice.
Bottom: Ice has a crystal
structure and when it forms
in air, the crystals can grow
into snowflakes, none of
which look the same.