Frost and Friction

The Science of Skiing

The team at the Centre for Sports

Engineering Research (CSER) at Sheffield

Hallam University were just the people to

explain a few things to me about sliding down

slippery slopes.

HE team at Sheffield have been

involved in major sports of all sorts

from badminton, golf and cycling to

cricket, pistol shooting and – the skeleton bob.

They designed and constructed a range of test

rigs and tools to evaluate the performance of

the famous Blackroc skeleton bob that Amy

Williams rode to a gold medal in Vancouver at

the 2010 Winter Olympics. It was Britain’s only

medal at the games, and a huge achievement for

everyone concerned. Nick Hamilton, a principal

research fellow at CSER and Senior Sports

Engineer seemed like the sort of man I should

be talking to.

T

The first thing that we had to bear in mind was

that we would be going for speed on snow, not

on ice, and, although ice and snow are both

essentially frozen water, they are entirely

different creatures. Solid ice, the sort that you

make in cubes in your freezer compartment

at home, forms when the temperature of a

large amount of water – a large amount being

anything from a single compartment in your

ice cube tray to a puddle or a lake – falls below

0° Celsius. Solid ice has a crystal structure

that occurs when the molecules in the water

rearrange themselves into a kind of framework.

In this form they are further apart than before

the water froze, so water is one of the few

substances that expand when they freeze. Most

things contract when they reach their freezing

212    world’s fastest sled

point. The fact that it expands means that ice is

not as dense as water, which is why ice floats.

Snow is also frozen water, but it forms in a

slightly different way. Instead of freezing as

a large mass of water, snowflakes form when

a tiny droplet of moisture in the air freezes,

usually freezing to a dust particle which

becomes the nucleus of the snowflake. More

droplets freeze on to the original one and the

snowflake, which is a kind of ice crystal, grows

until it joins with others or becomes so big on

its own that it falls out of the sky.

It’s easy to see, then, that the ice that forms

at ground level in a large amount of water

forms where there are lots of water molecules

pressing against one another. It forms under

pressure and is therefore far more solid than

snow, which is made up of snowflakes that

have lots of air trapped in the structure of their

crystal pattern and between each flake

or clump.

That’s all very well and explains what we
already knew – ice is hard and snow is soft –
but both are really slippery, or are they?

In fact, snow is really quite grippy stuff

compared with ice. You can easily stand on