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Homework Help: Friction coefficients

  1. Mar 20, 2008 #1

    Here is my question:
    should the kinetic friction coefficient Fk be the same when the object with mass m is pulled with a constant velocity as the kinetic friction coefficient with the same mass m when it was pushed and released.
    The measurements in the second case were taken from the decelerating part of the Force vs. Time graph.

    I have several table with data many of which i have calculated myself.
    from the formula: Fk = ;mu * Fn
    Fk - kinetik friction force,
    ;mu - coefficient of friction kinetic force
    Fn - normal force
    in the first case I have:
    m = 0.09 kg
    Fk = 0.1387 N (the uncertainty here is 17% !!! Fk = 0.1387 +- 0.0238)
    Fn = 0.882 N
    => ;mu = 0.1573

    In the second case:
    m = 0.09 kg
    Fk = 0.2585 N
    Fn = 0.882 N
    => ;mu = 0.2931
    here i also have uncertainty in acceleration which I use in order to find Fk.

    So, my question is: Should the coefficient ;mu be different in two cases or do I have such a big error?

  2. jcsd
  3. Mar 20, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    The coefficient should be the same. If you are pulling the mass be careful that no component of the force acts upwards since this will reduce the normal force.
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