10 Feb 2017

Surprisingly Slippery Science

Canada is a land of ice skating.  The longest skating trail in the world (according the Guinness World Records) is a 30 km trail around Lake Windemere in BC. Both our Women's and Men's Hockey teams have the best records in the world. Canadian figure skaters are an international powerhouse. So we really should know how a skate works. But do we?

Many of us have been taught that skates put pressure on the ice, which causes the ice to melt, and the skate then glides on a cushion of water.

It's certainly true that pressure decreases the melting point of ice. The pictures below show this. They're screen captures from an experiment shown in the National Geographic video at


Scientists place a wire with weights attached on top of a block of ice. Weights on the thin wire exert a lot of pressure on the ice.

The pressure melts the ice and the wire cuts through the block. The ice re-freezes above the wire as the pressure is released.

But there's a problem. The pressure of a skater is only enough to increase the melting point of ice by less than 1 degree. So how do skates work at -20 degrees?

A second explanation often offered is that the friction of the skate moving across the ice generates enough heat to melt the ice, and that creates the water for the skate to glide on. But that can't be the only explanation.

We've all had the experience of finding out that ice is slippery even when you're standing still. There's no need for friction to melt the ice to make it slippery.

The third answer was known by Michael Faraday as long ago as 1850, but somehow his views were largely ignored. There is always a layer of water on the surface of the ice. That's why two blocks will freeze together if you join them. The water layer occurs because the structure of the ice breaks down at the surface. The thickness of the water layer increases with the temperature of the ice.
Surface of Ice - from Wikimedia Commons

So pressure and friction make a small difference, but essentially skates slide on a layer of water that is always on the surface of the ice.


The technology of skates has changed a lot over the ages.

Steven G. Johnson (creative commons licence)
This picture is of medieval bone skates on display at the Museum of London. The accompanying label quoted a 12-century description of skating in London by William FitzStephen (Londoners would "fit to their feet the shinbones of cattle" and propel themselves with an iron-tipped stick).

This is an illustration of modern speed skating "clap skates". In my book Faster, Higher, Smarter you can find out how clap skates work and read the story of how this clever invention took a hundred years to become mainstream in speed skating.

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