When you are ice-skating you make use of friction in two ways – you glide forward and press sideways and outwards so that the blades of the skates cut into the ice in order to gain speed. But why do the skates glide so effortlessly on ice? Although this seems to be a simple question, it is only fairly recently that physicists have been able to provide a satisfying answer.
“We haven’t really understood why ice is so slippery, and there still remain some unclear aspects to the question,” says Johan Lindén, lecturer in physics at Åbo Akademi University.
He himself was once taught that the skate blade exerts enough pressure on the ice during skating to melt the topmost layer of the ice, but this has since been demonstrated not to be the case. Another mistaken theory is that it is the friction which melts a thin layer of ice and which is consequently diminished as the meltwater then functions as a lubricant. There might actually be some truth in that explanation, according to Lindén; but it is not the whole truth.
Recently it has been discovered that the outermost layer of ice is always made up of a thin ‘quasi liquid’. The topmost layer has no neighbours above it, and thus the water molecules can move more freely. Lindén compares the phenomenon to the surface tension of the top layer of liquid water. The layer is extremely thin, only 10–20 nanometres thick, but it is enough to make the ice slippery.
As skis have a larger contact area with the surface that is skied on, does that mean that they have less friction than skates?
“No, not at all. Friction is caused by two factors: a friction coefficient specific for the material in question and the weight of the burdening object, which is measured in Newtons. The fact that skates and skis are differently constructed is due to other factors, and I imagine that the way in which you sink into the surface is significant. That is to say, the size of the contact area does not affect friction. Skis do, nevertheless, always have a larger movement resistance in the form of loose snow which is pushed aside during skiing.”