Slippery ice

First – sorry blogging has slowed down a bit in recent days – busy time at the university having to teach courses and prepare for next semesters courses and get ready for a major conference all at once. So I’ll keep this one short.

The wild weather over the last couple of days has brought with it its fair share of slippery roads. Ice is nasty stuff when you are driving on it (or walking on it for that matter – I remember one January morning in Bedford in the UK when I didn’t even make it to the bus stop – the second time I’d fallen on the footpath after about 30 metres from my front door I turned back for the house to wait for the day to defrost a bit.)

What makes ice slippery? Most solids aren’t. However, ice is not your typical solid. One of its bizzare properties is that it is less dense than its liquid counterpart, water (hence ice forms on top of the water on a lake first of all). This property also means that, when ice is compressed (e.g. by a car or a foot) it melts.  Most substances would do the opposite – pressure would mean that a liquid turns to a solid. So we end up with a thin film of water on top of the ice, right under your car wheel, and, hey presto, there is no grip.

Ice skates are exceptionally effective at this – they have such a low area of contact with the ice surface that the pressure is very high, and thus the ice melts underneath exceptionally easily.  Skaters just slide along on top of a thin film of water.

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