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Homework Help: Friction Force FBD

  1. Apr 6, 2014 #1
    1. The problem statement, all variables and given/known data
    What is the greatest acceleration that can be generated by a runner if the coefficient of static friction between shoes and road is 0.95?

    2. Relevant equations
    Fnet = ma
    Force of static friction: usN
    Weight= mg

    3. The attempt at a solution
    FBD: Positive x axis is to the right. Positive y axis is to the top. Normal force pointing up, equal in magnitude to weight pointing down. Friction must point in the positive x direction.

    f = max
    ax = usg
    amax = (0.95*9.81)
    = 9.3 m/s2

    I just have one question. Does the friction force point to the right, in the direction of the positive x axis? Thanks!
  2. jcsd
  3. Apr 6, 2014 #2


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    To answer your one question yourself you can imagine trying to accelerate on an (almost) friction-free surface, such as ice.
  4. Apr 6, 2014 #3
    So the friction force would point to the right? Due to Fnet = ma, the acceleration and net force should be in the same direction. The only way this is possible is the friction force pointing to the right.
  5. Apr 6, 2014 #4


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    Yes, if the runner is trying to accelerate in the positive x direction (I didn't find that all too clearly in the story...). Runner pushes backward, exercises a force in the -x direction. No slipping means there must be a force in the +x direction to offset it: the friction force.

    Compare with car braking: friction slows it down. Accelerating: friction allows it to accelerate.
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