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Dry friction questions

  1. Oct 13, 2011 #1
    So I have been told that friction is caused by microscopic bumpiness in the surfaces of objects. Based on Friction= (coefficient of friction)x(normal force), if normal force is 0, then there is no friction. However, where is the line drawn between microscopic bumpiness causing friction and other things. Why cant the force of a nail holding up a picture not be considered friction? Or can it? Based on the above formula, the friction would be 0 though since the is no normal force perpendicular to the direction of friction. Is the formula a simplified version that is only accurate to a certain level of bumpiness and afterwards, its no longer considered friction? Also why is dynamic friction the same for all speed? Or is it? From my understanding, dynamic friction breaks bumps easier (due to more momentum) and/or does not all the to surfaces to come all the way into each other.

    Thanks!
     
  2. jcsd
  3. Oct 14, 2011 #2

    AlephZero

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    You are right that Coulomb's "law" of friction is only a simple approximation to reality. Rather than calling it a law, it is better to call it a model of friction (using "model" in the sense of a "mathematical model".) For hard objects, with moderate size forces between them, moving at fairly low relative speeds, it matches "real life" quite well. In other situations it can be very inaccurate. But "better" models of friction are usualy too complicated to use for hand calculations and need computer simulatons to use them, and therefore they are not very useful for teachng people the basics of friction. Unfortunately, some students get the wrong idea the the Coulomb "law" of friction is the whole strory, and it is a "law of physics" in the same sense as Newton's laws of motion of the ideal gas laws.

    But in your example, the nail is embedded in something flexible (e.g. a piece of wood) and there is a normal force acting over all over its surface, because the wood is trying to "spring back" to fill up the space where the nail is. To pull the nail out, you have to overcome the friction force created by that normal force.

    If the nail is horizontal with a picture hanging from it, there is also a vertical force acting upwards on the nail, equal to the weight of the picture, and that is another "normal force" acting between the nail and the wall.
     
  4. Oct 14, 2011 #3
    By the way, it's not just "microscopic bumpiness" that causes dry friction, but also molecular attraction due to the Van der Waals force. That is why very smooth surfaces can have surprisingly high friction coefficients.
     
  5. Oct 14, 2011 #4
    So what if the nail we by the wall just having a protrusion large enough to hold something up?
     
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