Tuesday, 7 December 2010

18 sleeps to go....

Well, today we have been having complications due to good ol' ice Ih.

We have been slipping and sliding all over the place but the question is why is ice slippery?

When I was at school and university, the long-held view for slipperiness was all about pressure and the density of the ice. Ice, unlike any other non-metallic solid, is less dense than its liquid counterpart (i.e. water). Because of this, when you stand on ice you exert pressure on the surface and the melting point is reduced creating a thin layer of water between the surface of the ice and the foot. This serves as a lubricant which causes the friction to decrease significantly and a slip to occur. Simple!

Now though, the view is that this answer is just a bit too simplistic. The melting point drops around 1oC for every 130 kg/cm2 which is a lot of pressure. I weigh 84kg and I have size 12 shoes. Running a quick measurement of each shoe, I have a total surface area of 552cm2 (gosh my feet are big) leading to a total paltry pressure exerted on the ice on which I walked today of 0.152 kg/cm2. My walking should have lowered the melting point of the ice by around 0.001oC. Given that the temperature was -8oC, why did I fall on my butt today?

Friction is one answer- the walking involves a degree of rubbing which creates heat and so increases the temperature of the ice (thus melting it and lowering the frictional constant due to the lubrication layer of water). I still think this is a little bit far-fetched since I am dubious that even the most vigorous walking would increase the surface area of the ice to melting point (if I remember correctly, to melt a piece of ice requires as much energy as it would take to heat the equivalent volume of water to 80oC).

I like the view that ice molecules that are in contact with air simply cannot bond properly with the molecules of ice beneath, leaving them in a semi-liquid state and free to move like molecules of liquid water regardless of the pressure exerted on the ice. Simply put,  ice is slippery because it is (very Ockham-esque).


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