Thread Tools
Old 12-25-2003, 02:19 PM
Guest
Join Date: Jun 2003
Posts: 5,724
Lowest and highest coefficients of friction?

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?
__________________
magnae clunes mihi placent, nec possum de hac re mentiri.
"Hubert Cumberdale, you taste like soot and poo." *cry*
disputed MPSIMS TMI award winner! and Also MMP TMI winner!
Old 12-25-2003, 03:27 PM
Member
Join Date: Aug 1999
Location: Alabama
Posts: 14,025
"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.
Old 12-25-2003, 05:56 PM
Guest
Join Date: Dec 2001
Location: New England
Posts: 3,055
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.
Old 12-25-2003, 07:39 PM
Guest
Join Date: Dec 2002
Location: Ontario, Canada
Posts: 2,097
I think the OP could be reworded slighly to ask simply "what material is the most fricitive (rubber?) vs. least fricitive (teflon?)"
Old 12-25-2003, 08:30 PM
KP KP is offline
Guest
Join Date: Sep 1999
Posts: 1,789
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.
Old 12-25-2003, 09:16 PM
Charter Member
Join Date: Aug 1999
Posts: 16,451
Low greased owl shit.
High Velcro
Old 12-25-2003, 09:32 PM
Guest
Join Date: Jul 2002
Location: Dammam, Saudi Arabia
Posts: 12,145
If I recall correctly, the lowest CF found in nature is wet ice on wet ice.

But then again I might be wrong.
__________________
800-237-5055
Shrine Hospitals for Children (North America)
Never any fee
Do you know a child in need?
Old 12-26-2003, 12:43 PM
Guest
Join Date: Mar 2000
Location: San Francisco, CA
Posts: 2,204
Quote:
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:
Quote:
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...
Old 12-26-2003, 01:40 PM
Guest
Join Date: Sep 2002
Location: Hampshire, England
Posts: 13,356
In military parlance, I believe the putative low-friction example involves manure and digging implements.
Old 12-26-2003, 02:57 PM
Guest
Join Date: Mar 2003
Location: [censored]
Posts: 5,338
Quote:
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.
Old 12-26-2003, 07:09 PM
Guest
Join Date: Oct 2001
Location: Cambridge. No, the UK one
Posts: 4,272
Quote:
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.
Old 12-26-2003, 07:28 PM
Charter Member
Join Date: Jan 2003
Location: Home of the haggis
Posts: 27,387
Quote:
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.
__________________
Quartz
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:04 AM.

Copyright © 2017
Best Topics: gunshot scar healed spelling joke football fanny pack lyrics innocent man skywalker fire department alumnus vs alum doctor reflector headband usps media diy fireproof box title for biography ethnicity vs culture living in filth cylon god deities 3.5 serial killer calendar roast mules drawing test turtle slavomir rawics colitas smell alternatives to dear gremlin rules jaw popped out simcity radiation yankee white facility christopher columbus book suicide headshot highest viscosity liquid define unsolvable amish inbreeding calvary or cavalry cat grunts what's the difference between pizza sauce and spaghetti sauce lighting a cigarette with another cigarette 17 year old dating 20 year old is it normal to shave your stomach how many slats for a twin bed what is inside guinness can man with long nails bugs bunny dogpile on the rabbit is there anything else you'd like us to know about you? feet itch like crazy scary noises at night stadium seat cup holders send money paypal without account do wisdom teeth keep growing liz brewer hang on sloopy les jeux sont faits translation shopko eye exam cost smell toast before a stroke what do restaurants do when you can't pay everything wrong with polar express can you say happy passover david roth fool us where to buy olympia beer weight of dry wall drive thru window height rainfall totals by zip code year to date bubble bath in a jacuzzi tub orange juice in a can what city was hill street blues based on why are siamese cats so vocal the golden turd american dad 2004 mazda 3 aux cord