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General friction formula's

  1. Sep 21, 2009 #1
    I was wondering how to modify F=ma and put in either static or kinetic friction coefficient. I assume it's something like F-(friction coef) but I'm just not sure, any ideas or websites someone can direct me to? thanks
  2. jcsd
  3. Sep 21, 2009 #2


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    Staff: Mentor

    When you draw the free body diagram, you put in a friction force vector as one of the force vectors acting on the object. The direction of the friction force vector is to oppose motion of the object (whether static or kinetic friction), and the magnitude is generally F = mu * N, where mu is the coefficient of friction, and N is the normal force between the object and whatever it is rubbing on. N is often just the weight of the object, for example.
  4. Sep 22, 2009 #3
    Not the object as a whole. Only the motion of the contact point. For the FBD of a rolling wheel or an entire car for example, friction can turn out to be in the direction of motion.
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