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Quizzing the Anonymous
Ignoramus et ignorabimus
Slippery slope 
19th-Aug-2009 08:46 pm
thinking
Why is ice slippery? When I was a kid, the textbook answer was pressure melting. This answer is incorrect. Then it was said that the correct answer was frictional heating. Then it was a film of liquid or disordered water on ice. I remember Gabor Somorjai announcing this discovery to the applauding audience: the problem has been solved, mission accomplished. After 25 years of exploring this layer, no consensus has emerged as to its properties or even its thickness.
http://www.lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf

The latest papers tell that the liquid film is too thin to explain why ice is slippery, so it must be frictional, by default.
http://prola.aps.org/abstract/PRB/v61/i11/p7760_1
http://www.uoregon.edu/~rempel/12_RMP06re.pdf
Others still maintain it is viscoelasticity of the liquid film
http://pubs.acs.org/doi/abs/10.1021/la9001994

The answer is surely unknown. I wonder if any aspect of friction is truly understood, even the fact that the force is proportional to the load. Most theories tell it really shouldn't except for the hardest materials.
http://www.tau.ac.il/~klafter1/ar3.pdf
http://www.phys.appstate.edu/coffey/nano/nanotribologyencyclopedia.pdf
Every time I hear a talk on friction, it is painfully obvious how little is known about it. One would think it should be much harder to explain the formation of a star or prove the Poincare conjecture than explain slippery ice. But it is just the opposite. The problems that look hard are first to go. The problems that look easy are the ones that stay without any hope of resolution. A mathematician friend of mine told me the same: if it is a complex conjecture with a page of qualifiers alone and looking positively impossible to prove, chances are it will be solved quickly. The hardest problems come in the simplest form; that's where you waste your life with almost no progress. I vouch the same for every field I've worked in. It is not complex that is hard, it is simple that kills you. We often do these complex things out of sheer frustration at our inability to tackle the simplest thing. I know I do, and I've observed it in the others. I am also convinced that the real goodies are there, in these simple problems.

Why is it the simplest question that is invariably the hardest and most intractable one? Is it the law of Nature, a fundamental flaw of mind, or what? I have thought about this question long and hard, and I do not know the answer. No one seems to. It must be one of those intractable simple questions. But if this one is answered, maybe we'll be able to answer all of them?
Comments 
20th-Aug-2009 03:49 am (UTC)
I think you may want to have a discussion with duchifat, he knows something about friction.
20th-Aug-2009 04:28 am (UTC)
When we talk about explanations, many other phenomena are similar to friction. The clue is simple. It is not the nature of friction that matters. It is the nature of explanation.

What do we mean when we say that something is explained? Very often (not always) we mean that explanations are something that is more or less reduced to our common sense or natural experience. Of course, those are not true explanations. True explanations are impossible.

Natural sciences produce 'reasonable explanations', but it is not explanations that make science. Sciences allow us to predict and to make use of phenomena. Explanations are mere by-products.
20th-Aug-2009 01:49 pm (UTC)
Premelting and its role in friction are general. If I reformulate this question as: what is the general mechanism of friction for crystalline solids slightly below their melting point, it is as intractable as the specific question about ice. You can reformulate my question, why it is so hard to predict rather than explain. I think it is also incorrect on its face: to predict, you need to have mechanistical understanding. Especially in friction, where predictions by analogy do not work.
20th-Aug-2009 03:44 pm (UTC)
I see your point. Or at least I think so.

All I want say is that there are many other situations where predictions, which are based on common sense or our previous experience, appear not to be valid.

The simpliest example is the strange wide-spread belief that heavier objects fall faster (when the influence of the air can be neglected). Galileo was not sure about that either. He had to carry out some experiments.

Friction is a tricky thing. But other phenomena are also tricky.
21st-Aug-2009 12:45 am (UTC)
In real life, the influence of air can never be neglected, and heavy objects do fall faster.
21st-Aug-2009 01:30 am (UTC)
I can tell what the answer might be, but I am hardly original there. We are conditioned to think that complex things have complex causes whereas simple things have simple causes. That's how our minds work; perhaps there is some cognitive advantage in believing such things, especially judging the behavior of people and animals. But, by and large, this is not correct about the majority of important things in the physical world and mathematical objects. Complexity often arises through the interplay of simple causes. Complexity is deductive, whereas simplicity is inductive. With some effort, the simple rules leading to complexity can deduced, ans so it can be explained and predicted. By contrast, simplicity is the emergent property of complex systems. As such, it is difficult or next to impossible to explain. Furthermore, such explanations are rather pointless because the motif is more basic than the interplay of the complex causes providing this pattern to which it is being reduced. The causes may change, but the pattern remains; so it is more fundamental than the particular implementation. Robert Laughlin argues for these kinds of ideas in his book, "A Different Universe", with many examples from solid state physics.
21st-Aug-2009 05:29 am (UTC)
Objects with similar aerodynamic properties do fall at an equal speed irrespective of their masses. The examples are numerous. (Of course, we are not talking about objects like airplane wings.) Air can be neglected in many cases.
22nd-Aug-2009 05:44 am (UTC)
Try dropping a paper ball and a steel ball, and see which one falls faster. Their aerodynamic properties are the same.
22nd-Aug-2009 06:01 am (UTC)
Please stop finding faults with my words. :)
Instead try dropping a wooden croquet ball and a steel ball.
22nd-Aug-2009 06:12 am (UTC)
If you drop them from as little as 30-50 m, there will be noticeable difference.
22nd-Aug-2009 06:16 am (UTC)
By the way, I don't remember how tall the tower in Piza is, but the texbook descriptions of Galileo's results seem to be impossible: he should've noticed the difference, too.
22nd-Aug-2009 06:23 am (UTC)
Just checked: what he did was not dropping light and heavy objects of the same size, but dropping objects of the same shape and material, but different size. Actually, according to Wiki, he didn't drop anything at all - it's just a legend.
Sorry, I have to drop out of the conversation for a few days - will be away from internet access.
22nd-Aug-2009 06:32 am (UTC)
Once again, I do not say air does not matter. It does. To be more correct, sometimes it does, sometimes it does not. :) We are disputing over a thing that is obvious for both of us, aren't we? :)))

By the way, some people say Galileo never dropped objects from the tower in Piza. It looks like a fiction. But I do not insist on that. :)
20th-Aug-2009 04:41 am (UTC)
If watery film was the cause, than ice would be much more slippery at -10C than at -60C. In my experience, it's not. It gets more slippery above -1C, but below that it seems not to depend on temperature.
I can't remember if "dry ice" is also slippery. It can't have liquid film, can it?
I think it might have more to do with crystal structure.
20th-Aug-2009 01:43 pm (UTC)
No, it is actually gets less slippery at lower temperature; below -35 C you cannot skate at all. Dry ice is not slippery, but diamond is more slippery than ice without premelting. It is everything, including the lattice, the properties of the surface - in fact, nearly all aspects of it excluding the one that everyone believes controls friction: the roughness of the surface.
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