Skating on Water

Dr Em’s fun fact for Friday 13th: whenever you go ice skating, you actually skate on water.

 

Let’s break this down. Remember how your mum used to tell you that ice skates could cut people’s fingers off? That might be just a scare tactic or an excuse but there is some logic behind it. It’s all to do with the fact that ice skates have a very very small area that’s in contact with the ice. That means a very high pressure.

If we say that the ice-skate blade is 25cm long and 1mm wide, we can work out the total area in contact with the ice.

A = 0.25 x 0.001 = 0.00025 m-squared

Pressure is Force divided by Area so presuming that you weigh 60kg let’s find out the pressure. Assume gravity is 10 m/s/s

F = weight x gravity = 60 x 10 = 600N

So,

P = 600 / 0.00025 = 2400000 Pa = 2.4 MPa

That’s quite a large amount of pressure, certainly enough to melt the localised patch of ice underneath your blades. Of course, it freezes over again more or less as soon as you’ve passed, but the fact remains that you ice-skate on water.

As for fingers, I think we need some volunteers to see whether 2.4 MPa is sufficient to remove one of those…any takers? On second thoughts, that sounds too much like Biology for me.