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chaoticbear
12-25-2003, 02:19 PM
Basically self-explanatory. Is there some extreme value that either can be (besides lower limit=0), and if so, what are they? Also, what are the highest and lowest that we've been able to produce?

scr4
12-25-2003, 03:27 PM
"Coefficient of friction" isn't a rigidly defined physical quantity, at least I don't like to think of it that way. It's just an approximation; if you approximate the force vs. friction as being directly proportional, that proportionality constant is the "coefficient of friction." In reality, things don't behave so nicely.

If you want examples of materials with extreme coefficients of friction, Teflon and rubber are commonly available examples of each. If you allow systems rather than raw materials, you can easily rig up a zero-friction surface by using magnets or air cushion to make an object float. You can have arbitrarily high "friction" by using a velcro-like structure.

I know, this is a nerdy and not very useful answer. I guess what I'm trying to say is that the question is not as self-explanatory as you seem to think.

Hyperelastic
12-25-2003, 05:56 PM
The problem isn't lack of precision in the definition. Because the coefficient of friction is defined theoretically (or operationally if you like), it is as precisely defined as anything. Put a block of material 1 on a ramp of material 2. Slowly incline the ramp. The tangent of the ramp angle when the block starts to slide is the coefficient of static friction.

As scr4 states, the idea that two contacting bodies have a fixed ratio of friction force to normal force has a limited range of usefulness. Like many theories in applied mechanics, "it works when it works." It doesn't work well when fluids are involved; the viscous friction force tends to be proportional to the relative velocity.

The only theoretical limit on the coefficient of friction is that it can't be negative. If it were, I could build you a very useful machine.

Trigonal Planar
12-25-2003, 07:39 PM
I think the OP could be reworded slighly to ask simply "what material is the most fricitive (rubber?) vs. least fricitive (teflon?)"

KP
12-25-2003, 08:30 PM
Actually quite the opposite: given that we are dealing with extreme values, They must to specify both surfaces. At extremely low or high values of friction, nonclassical effects are very significant. One-ended coefficients of friction only apply in the classical regime.

IIRC, 'wet ice on ice' or even 'dry' ice-on-ice at -180C had a lower pairwise coefficient of friction under many conditions than 'teflon on ice' [etc.], though some low friction mat'ls have a lower one-ended coefficent of friction than dry ice by standard measurement.

Here's a link to an example of the nonclassical friction effects of ice (http://lbl.gov/Science-Articles/Research-Review/Magazine/1997/story8.html).

Rick
12-25-2003, 09:16 PM
Low greased owl shit.
High Velcro

Paul in Qatar
12-25-2003, 09:32 PM
If I recall correctly, the lowest CF found in nature is wet ice on wet ice.

But then again I might be wrong.

DarrenS
12-26-2003, 12:43 PM
Originally posted by Hyperelastic
The only theoretical limit on the coefficient of friction is that it can't be negative. If it were, I could build you a very useful machine. Don't be so sure - from here: (http://sciam.com/article.cfm?articleID=0002C309-F7DB-1FD3-B58083414B7F0000&catID=2)Negative friction, which would cause molecules sliding past one another to speed up rather than slow down, might be possible. Behind the theory is the van der Waals force, which normally causes molecules to weakly attract one another...

Colophon
12-26-2003, 01:40 PM
In military parlance, I believe the putative low-friction example involves manure and digging implements.

emacknight
12-26-2003, 02:57 PM
Originally posted by Paul in Saudi
If I recall correctly, the lowest CF found in nature is wet ice on wet ice.

But then again I might be wrong.

I agree wet ice is low, senovial joints (ie your knees) have an insanely low coefficient of friction that I believe is lower. The senovial fluid is polarized with respect to either side of the joint and is remarkable under load. One estimate put it at less than 0.001 (from a course I took in biomechanics).

On that same note, the lowest would be two magnets (of the same polarity) as mentioned above, and I believe the highest is also two magnets (of different polarities).

I'm not sure if Velcro(R) counts since to me it seems a bit to large, like saying two gears meshed together. On a microscopic scale surfaces look like little jagged teeth that grind together and create resistance. The smaller the teeth the lower the friction, and from what I remember there are calculations that can be made.

I think for the point of the OP, the ice example and the senovial fluid might be considered lubrication and hence cheating. As is the influence of magnetism.

As for highest, I'm going with rubber on rubber. Lowest would be certain plastics that again make use of polarity to slide relative to eachother. But their actual coefficients of friction would depend on surface area in contact.

Shade
12-26-2003, 07:09 PM
Originally posted by Hyperelastic
The problem isn't lack of precision in the definition. Because the coefficient of friction is defined theoretically (or operationally if you like), it is as precisely defined as anything. Put a block of material 1 on a ramp of material 2. Slowly incline the ramp. The tangent of the ramp angle when the block starts to slide is the coefficient of static friction.Nitpick: can't it be greater than 1? Then this experiment wouldn't find it - it would just 'stick' at 90deg. But I agree a slight change to your exp defines static friction perfectly satisfactoraly.

Quartz
12-26-2003, 07:28 PM
scr4 wrote
If you allow systems rather than raw materials, you can easily rig up a zero-friction surface by using magnets or air cushion to make an object float.
But is there not some friction here? After all, the intervening air gets hotter.

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