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Why Physicists Don't Like To Talk About Friction
fm6 writes: "You would think that force required to overcome friction would be a function of the area of contact. But according to this Scientific American article, that's not true, and physicists don't have a really satisfying explanation." This is the sort of article that makes you want to go experiment with those teflon-coated disks made for moving furniture.
Three hours and not a single post.
Guess they don't like to talk about it.
k.
"In spite of everything, I still believe that people are really good at heart." - Anne Frank
I only stumbled across this link on the sidebar... why wasn't it shown in the main news body?
I guess friction isn't a burning topic for physicists nowadays.
where theres friction, theres a good sex joke...
I guess someone should post something serious.
This moving-crack theory is crap. I can't show it's not true, but a model of interlocking surfaces explains friction perfectly well. Consider two horizontal surfaces whose interface is a zig-zag. There is a force Fd holding these surfaces together and a horizontal force Fm on the top surface. The top surface will not move until it slides up to the peaks in the lower surface. It's quite trivial to show that the required force depends upon the degree of interlocking (the angle of the zig-zag) and the force Fd, which must be overcome to seperate the surfaces.
It doesn't seem mysterious to me that it's related only to the force. The same force distributed over a wider area actually applies less force per square [your measure here]. So it's a wider area - big deal. It's compensated for by a proportionally smaller force per square area. Whatever atomic force is working at keeping the surfaces distinctly separated has to do less work at any single point when the force is acting in more places. The net effect? Surface area is irrelevant. Am I missing something? Is this explanation just way too simple? What's the catch?
Paradise is exactly like where you are right now, only much... much... better. - Laurie Anderson
I think that certainly explains the static force of friction F sub s, but what of F sub k? Why should F sub K typically be so much higher than F sub s?
I suppose one might argue that a surface that experiences an F sub k might be assumed to have previously been at rest, and nestled firmly in the lowest state, or the trough if one likes, but why would this be SO much higher (typically)? Interaction between electrons at the surfaces? If there is a limited amount of interaction taking place, or the formation of weak bonds, why not view it as analogous to a crack (a very well understood phenomina)?
But back to F sub s, the static force of friction, wouldn't the surface fall foreward into the troughs of the supporting surface some of the time providing a slightly accelerating force of friction which would then turn decelerating as the atoms being supported tried to move up out of the trough against the force of gravity? Of course, that's not what we see, so it can't be the complete picture.
Why not move back to the formation of tenuious bonds between the surfaces (for a moment). If these bonds are being made occasionally, then stretched and broken, it would seem to my mind's eye that for a macro sized object F sub s would likely be a near constant (surface irregularities, pressure, whatnot would all play a part). Since the breaking of these bonds in a sence does change the surface properties, why not view it as a moving crack if it is convienent? Certainly we all except greater abstractions than this in our everyday life, if some scientists find it a helpful model is it worth belittling? Sometimes abstractions like this, reguardless of their accuracy, can be surprisingly useful. For my part, it is consistant with what I know to be true and seems to do a better job of explaining, at least for me, better than a classical speed bump theory. Your milage might vary; but so might theirs.
--Jimmy has fancy plans; and pants to match.
I find those work even better if you coat the entire floor with Mobil 1 first... add a pump and filter, and you never have to vacuum, either :)
"It's tough to be bilingual when you get hit in the head."
Another place to read about this (complete with MPEGs of the self-healing crack) is at The PhysicsWeb.
Wider car tires work better because the rubber is less likely to tear due to the force. When stopping or starting, the force on the car tires is often enough to tear off rubber (hence the tracks you leave on the road). This means that the limiting factor in tire traction is not the actual coefficient of friction, but rather the strength of the tire. (Because we are sliding due to tearing rubber *before* we run exceed our force of friction) Since the strength of stuff *is* dependent on area (think 2x4s vs. a broomstick), wider tires will not tear as quickly, meaning more of the friction is available before we slide.
m l
So it's not the friction that's changing due to area, but how quickly the tire tears.
For a reference (quick search on google) see:
http://www.cosm.sc.edu/~phys153/tirefriction.ht
Shut the fuck up, man. Don't be talkin' about it. Don't even talk about not talkin' about it. Now you've got me talkin' about it!!
Friction generates electricity, and other magnetic beings too, before going on atomic scale, and before you go on these argues please don't forget gravity, which does a particular behaviour.
Nowadays science cannot 100% assume gravity is any type of particle -wave-, like quantum theory, or if it's a single modification caused by presence of mass in time-space, like it was stated in Einstein famous theory.
This is rediculess, but there's also a little influence over "classic mass" friction, and it should be considered into quantum-based models, which were extensively used by the scientific community.
as a student here at UT, its nice to see some credit for our profs as one is mentioned in the article, also Dr. Pennebaker, world renowned psychologist. Very interesting shiste. Look him up.
Stuff thats less than flat... Like carpet has a ton of little hills or those fibers coming out of it. I just assumed that when another object rested on the floor, that some interlocking between the hills/valleys of each objet occured. When you pushed the object on top of the floor, a bit of force is needed to get up out of the interlocked state, but once you're moving, kinetic energy keeps you from falling into a deeply interlocked hill/valley state.
Kinda like when you're driving your car. If you hit a really deep pothole right, your tire may not fall the whole way in...But if you park your car, and the tire falls in, it will become more locked... Bad analogy, but thats how I always assumed it worked.
God spoke to me
Think about it. By simple everyday experence we know that friction is proportionate to the force of contact (typically, the weight of the load). So if you have to drag two identical crates, which are currently stacked one atop the other, and want to reduce the friction you can remove the top crate, cutting the friction in half. Note that you haven't affected the area of contact at all.
Now suppose you decice to move both crates at the same time, but not stacked. Each crate will have half the friction of the original load, and thus the whole will have the same friction, even though we have doubled the area of contact.
This is in the same category as the "you'd think lead bricks should fall faster than iron bricks of the same size and shape" or "which weighs more, a pound of lead or a pound of feathers"?
You might assume that friction was proportional to the area of contact, but you wouldn't think it.
-- MarkusQ