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Homework Help: Block on Ramp

  1. Dec 20, 2009 #1
    1. The problem statement, all variables and given/known data

    A 15.0 kg box is released on a 30 degree incline and accelerates down the incline at .30 m/s^2. Find the firctiopn force impeding its motion. What is the coefficient of kinetic friction?

    2. Relevant equations

    simga F = ma
    Force of friction = MU F_N

    3. The attempt at a solution

    SIGMA F_x = m a_x = F_g_x - F_fr_k = mg sin THETA - mg cos THETA MU

    therefore

    mg cos THETA MU = mg sin THETA - m a_x

    I divided through by mass

    g cos THETA MU = g sin THETA - a_x

    I do not see what is wrong with this solution. This gave me 4.6 N. I'm suppose to get 69 N. I know that if I muliply 4.6 N by 15 (the mass that I canceled out) I get 69 N.

    So apparently I don't know why I can not divide by the mass here and would like to know why...

    Surpisingly enough when I rearanged for Mu I did this and got the right answer according to the back of the book

    F_fr_k = 4.6 N = g cos THETA MU

    therefore

    MU = 4.6 N/(g cos THETA)

    this gave me the right answer of .54

    THANKS!

    Those are subscripts by the way

    F_fr_k
    is the force of kinetic friction

    and the all capital letters are one varialbe and are greek letters thansk!
     
  2. jcsd
  3. Dec 20, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    There's nothing wrong with dividing by the mass when you're solving for μ. But the first part of the question asks for the friction force, which is μmgcosθ, not μgcosθ.
     
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