Ice Is Slippery!, Why?

I've got a question that I hope someone can resolve. A little background on why I'm asking first. I write a hockey blog covering the University of Alaska Anchorage Seawolves. There's a bit of a break every year during the holidays and not much to write about as a result.

I'm considering a post to educate my readers as to the physics of the game. Most of what I'm thinking of explaining can be done with f=ma -- i.e... shooting the puck, hitting your opponent and the like. However, I have two competing theories about the most important aspect of the game: Why is ice slippery?

On one hand is the explanation that that the pressure exerted by a skate blade on the ice creates an area of melting at the point of contact which facilitates the skates movement (this seems to be a "traditional" explanation). On the other hand I recently read that the water molecules on the surface (as opposed to the core) of the ice are more loosely bound and that arrangement facilitates the skate blades function. I hope I've characterized those two theories correctly.

My bit of web research (and my inability to discern the truth between the two) isn't any help so I'm turning here hoping for something definitive I can use.

I tend to lean toward the loosely bound molecules as the best answer because the first explanation in no way accounts for the slipperiness of a puck which weighs very little and contacts the ice over a much larger area relative to a skate blade. But I'm also confused as to why slipperiness is necessary for the sharp skate blade to work. In some sense wouldn't the skate blade require a certain resistance to be effective? Is the ice both slippery and resistant?

Anyone here know anything about ice?

This post has been edited by uaafanblog on Dec 19 2007, 06:17 PM

Well, I can't add much from the physics standpoint, but having played hockey for a few years I would say that forward thrust is generated by pressing back on the edge of the blade (or at an angle, like tacking in the wind), so friction (or lack of) isn't really the issue. Some materials are naturally slippery (i.e., teflon), likely because the surface of the molecules fail to interact with other molecules because they are 'satisfied' with their charge state (i.e., the partial charge of florine (?) atoms exposed on the surface of the molecule is exactly balanced with the partial charge of the atom (carbon?) it is bound to). Perhaps ice is like that as well. I also have read that the temperature of the ice impacts its 'skatability' and ice can be too cold for optimum skating. I have also seen people skate on artificial ice (some sort of plastic) and they also appear to use the side of the blade to push (or toe pick).

I have read in other places that wet ice on wet ice is considered the most slippery and all measurements are made against that standard (obviously there must be some standard temp and pressure). I have also found in various reading that as the hardness of a substance increases its coeficient of friction decreases. Not sure why that is, but I do know that in high-end watches the 'jewel' movement wasn't just for kicks, it also allowed them to reduce the amount of oil they had to add to the parts, thus increasing the service intervals.

I can tell you that skis work because of the water layer created by friction of the ski over the snow. (The layer freezes right after the ski moves past it). This water layer is the reason why you need ski wax and ski bases have "structure" (tiny grooves) rather than being completely flat. Snow itself is quite abrasive.

Beats me about ice, but that may be part of it. The shear smoothness (particularly of contaminated ice) by be another.

I'd tend to go with this:

http://en.wikipedia.org/wiki/Ice_skating#How_it_works

Feel free to ask questions if it isn't clear.

Since starting this thread (and because of additional reading that I should have perhaps done before starting the thread), I've come to the conclusion that of the two competing theories that it is indeed the second that is responsible for ice being slippery.

So to summarize (and forego the need for any further responses) the post on my blog will read something like this:

(a preamble explaining that the long held theory of pressure causing melting is simply not true then onto the "real" reason below)

Ice is slippery because the lattice-like structure of the crystals (which gives the ice rigidity) is somewhat mysteriously absent at its surface. Instead there is a "quasi-fluid layer" that exists at the surface. Physicist's have difficulty explaining how and why this situation exists. It clearly isn't a function of temperature since this "quasi-fluid layer" exists at as low as -183F. But even at a relatively moderate temperature of -60F one would find it very difficult to skate efficiently. Apparently, the boundry layer of many (not all) "solid" objects differ from the underlying layers of the object. In ice's case physicists found that the bonds of hydrogen atoms were missing from the lattice structure. Why you ask? To be honest, the mechanisms of the different states of water are surprisingly questioned at the highest levels of physics. We all know that ice melts, but did you know that physicists are only now beginning to understand and record the mechanisms that are responsible. Sure sure you say ... ice melts because it gets warmer and the molecules change state from solid to liquid. Yeah ... you're right in the simplest sense. But follow a couple of the links I've provided and you'll discover a complexity that surprised the hell outta me.

I'll provide the following links inline for my readers:
newscientist.com

livescience.com

Wrong!!!!

Samorjai Rocks!

Like I said ... I should have done a bit more reading before starting the thread. Even though there "appeared" to be competing ideas there really aren't. Thanks to those who responded. Additional suggestions as to the exact text are appreciated but remember my readers are "hockey fans" ... nuff said?

This post has been edited by uaafanblog on Dec 21 2007, 01:01 PM