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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
If you assume that each surface is a series of circular arcs, instead of a zig-zag, then you get a similar result for static friction. If the surfaces are already moving, then they can't interlock as much as when static, and so the tangent force resulting from the normal force is reduced from that required to start from rest. But then this predicts that the sliding friction is a function of the extent of interlocking during movement, which depends on velocity. Oh well.
Bugrit! Millenium hand and shrimp!
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