Skating Shoes and Ice: Uncovering the Mystery

[Sarahchichi]

I want to know why we can easily slide on ice when we wear skating shoes. Originally, I guess it is due to the friction between the metal and the ice. But I think it is not the correct answer. Could anyone help me with this problem.

Thanks a lot,
Sarah

 

[whozum]

Thats pretty much it, the blade makes a really small contact area with the ice, since its also a thin blade it cuts into the ice a little, but I don't know how that helps.

Also, (combed) ice has a very low friction coefficient.

 

[Sarahchichi]

When I think deeper into the matter, isn't it because of the heat produced by friction that melts the ice? Then the liquid (water) acts as a lubricant to reduce friction.

 

[whozum]

Well, I would think that if your skate is melting the ice as your skating along you arent encountering liquid ice since you've already passed over it. I'm sure there is a deeper explanation, but I couldn't help you with much more than I've said.

 

[brewnog]

Hmm, I always thought that the pressure was sufficient to cause melting of the ice, lubricating the skate/ice interface. I've just discovered it's a myth.

Recent studies show ice at the surface is not the same as the structure of the bulk. It has some of the character of water because not all of the hydrogen bonds are satisfied. It appears that this more subtle effect accounts for the low coefficient of friction of icy surfaces.

http://www.physlink.com/Education/AskExperts/ae357.cfm

 

[Yegor]

I'm absolutaly not sure, but I have an idea. What is the difference between skating and ordinar shoes? Skating shoes have very thin surface. Why do we need it? May be, because of men's body must be inclined (refering to ice's surface). It's like a bicycle on the arc. When we slide each step we are moving across the arc, aren't we?. Then there appears a net force which is directed to the centre of circle. But if we are moving across the straight line a have no ideas (realy, how we can preserve equilibrium in this way?).
I hope, my english is understandable.

 

[Yegor]

http://www.exploratorium.edu/hockey/skating1.html
It's useful

 

[OlderDan]

http://www.physlink.com/Education/AskExperts/ae357.cfm

http://www.exploratorium.edu/hockey/skating1.html
It's useful

I don't find the calculation in the first article very satisfying. The assumption that a blade is about 1/2 cm wide for calculating the pressure is way off base. Skate blades are ground with a hollow between the edges to limit the contact surface to just the edges of the blade. It is not the width of the blade times the length. The conclusion that the calculated pressure is only sufficient to raise the temperature a fraction of a degree is unsupported in the article, but I do think it is correct for the calculated pressure. It appears to be based on the phase diagram for water which shows that it takes an enormous amount of pressure to change the melting temperature slightly. I seem to recall this slope is found from the Clausius-Clapyron relation involving the latent heat and change in volume at the phase transition. For water it is about -1.23 x10^7 Pascals per degree C. But there is no account taken in this article about the motion of the blade relative to the ice. We all know that if your hands are cold and you want to warm them up you don't just press them together. You rub them to generate heat. There is no consideration of heat generated by the relative motion of the blade and the ice in this analysis.

The second article describes an investigation into the nature of the surface of the solid, which for any solid has to be different from the bulk material. You cannot expect the bulk properties, where every mollecule is completely surrounded by like molecules, to be the same as the surface properties. Which raises the question of how can you expect to refute the idea that ice skates melt ice at the surface by doing a simple pressure calculation that applies to the bulk material.

I think both articles make the point that there is something about the surface of ice that is different from the bulk properties that is somehow connected to reducing the friction of ice skates, but neither of these articles refutes the idea that the ice under the blade is melting. Perhaps the conclusion to be drawn from the second article is that it is a lot easier to melt a little bit of ice at the surface by applying pressure to it than one might predict by considering the application of pressure to the bulk material.

The question of whether the ice melts under the skate would best be answered by looking for water in the skate trail immediately after the skate has moved on, or for a change in the surface . Why is it that on fresh ice you can always see a trail of the skate? It is because the ice that was there is not where it used to be, even if the skate is moving perfectly straight. The ice has not been scraped away as it is when the skates move sideways. Where did it go? If you get an ice cube out of your freezer and run a knife over it, the knife leaves a depression, even if the pressure is very small. It leaves a depression because the ice melted under the blade. You can see it and you can feel it. The ice that was there is gone.

The point is, neither of these articles debunks the "myth" that friction is reduced by a layer of slippery water under the skate blade. What may well be true is that you cannot account for that melting based on a simple pressure calculation relating to the bulk properties of ice, but you might very well account for it with a better understanding of the nature of the ice surface.

 

[whozum]

OlderDan should come with a set of cliffnotes :)

 

[Sarahchichi]

thank you all and the website