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Theory Under Pressure Skates on Thin Ice

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The Deceived Wisdom: Skaters can skate on ice because their skates melt the surface of the ice, creating a thin lubricating layer.

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Countless science teachers, textbooks and fans of ice skating will tell you that ice melts under pressure. They explain how applying pressure lowers the freezing point of water so that it has to be much colder before it will freeze into solid ice, and how, conversely, ice under pressure will melt.

The classic example of this phenomenon in action can be seen every time a skater's blades swish across the surface of an ice rink. The relatively sharp edge of the blade and the weight of the skater pressing down on the ice lower its freezing point so that the ice beneath melts, forming a thin film of liquid water on the surface of the rink - across which the skate can then glide with almost no friction.

Unfortunately it is simply not true.

Scientists have calculated the change in the freezing point of water at different pressures and backed it up with experiments. To lower the freezing point of water from 0°C to -1°C, you must apply a pressure more than 121 times the pressure of the atmosphere bearing down on your head right now.

One of the scientists who has done the watery sums, chemist Kevin Lehmann of the University of Virginia, has a solid answer. He started by assuming that an ice skater weighs about 75 kilograms. The blade of each skate in contact with the surface is about 3 millimetres wide and 200 millimetres long. Pressure is defined as the force applied to a specific area. The 58 force pressing down on the ice is 75 kilograms multiplied by gravity, which has a value of 9.8 newtons per kilogram.

So, a force of 735 newtons (75 multiplied by 9.8) is pressing down on the blade of that skate. To calculate the pressure we need to know the area of the blade in metres. 3 by 200 millimetres is 0.003 by 0.2 metres, an area of 0.0006 square metres. The pressure is the force, 735 newtons, divided by this area, which is 1,225,000 newtons per metre squared, or 1,225,000 pascals.

That value for the pressure being applied through the skate to the ice sounds enormous. And in some senses it is: it is about twelve times the value of atmospheric pressure, and it's the pressure you experience if you dive under water to a depth of about 120 metres. But this pressure is about ten times too small to melt ice. The skater would have to apply a pressure of 120 times atmospheric pressure to do that, and to exert that amount of pressure they would have to weigh ten times as much as a normal ice skater, and so be about 750 kilograms.

You might be wondering whether the pressure could be increased by sharpening the blades. After all, dividing the force of a 75-kilogram skater by a smaller number would equate to a higher pressure. But the effect would be to lower the freezing point of the ice only by a few tenths of a degree. Given that most ice rinks freeze their ice to well below 0°C, this would have little impact. The ice would stay solid.

So how do skaters skate over the solid and rough surface of ice if there is no liquid lubricant in the form of water to allow them to do so?

Lehmann concedes that, as with many other properties of water, scientists simply don't know. There are theories about the water molecules at the surface and how they are not being held as tightly in the ice as those within the frozen solid. There are also ideas about defects in the structure of ice that might allow some water molecules to become loose and so enter the liquid state.

It might be that the steel of the blade somehow grabs these loose water molecules and promotes melting as more and more water molecules loosen their grip on the ice to form that thin slippery layer of water below the skate.

Either way, this has nothing to do with the pressure applied.

One property that quickly becomes apparent to anyone new to ice skating, however, is that when you land on the ice with a bump and struggle back to your feet, your body heat allows the frozen particles of ice to quickly revert to the liquid state ... leaving you with a soggy behind.

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The Science: Applying pressure to ice has the effect of lowering its freezing point, which means it will melt to form liquid water above a certain temperature. However, the pressure exerted on the ice by even the bulkiest of skaters will be a fraction of that needed to melt ice at the frozen temperature of an ice rink.

Find out more about all the things you thought were right that are wrong in my book Deceived Wisdom available from the usual outlets and for immediate download from the Sciencebase web site.

You can also download an extended sampler of the book here.