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Homework Help: Calculating frictional force?

  1. Oct 30, 2009 #1
    1. The problem statement, all variables and given/known data

    Three identical boxes sit on concrete floor, each box weighs 30 lbs (133 N). Each has a coefficient of static friction of 0.74 and a coefficient of dynamic friction of 0.64. Horizontal force of 80 N is applied to Box 1, horizontal force of 100 N is applied to Box 2, and horizontal force of 120 N is applied to Box 3.

    a.) Do any of the boxes move?
    b.)Calculate the actual frictional force elicited between the ground and box in each case. If the boxes move, compute the dynamic friction that resists the sliding. If the boxes do not move, compute the static friction that resists the applied force.
    Note: frictional force is not always the maximum.

    2. Relevant equations

    a.) F = ma
    RNormal- W = ma
    RN = W

    Fmax = [tex]\mu[/tex]s (RN)

    b.) not sure

    3. The attempt at a solution

    a.) I used RN = W to determine that RN = 133 N

    Then I used Fmax = [tex]\mu[/tex]s (RN)
    Fmax = 0.74 (133)
    Fmax = 99 N, the force required to move the box

    So Box 1 would not move, Boxes 2 and 3 would move.

    b.) This is where I am most confused, I am not sure how to calculate the static and dynamic friction, but I tried anyways:
    For Box 1: static friction = [tex]\mu[/tex]s (Fapplied)
    =0.74 (80 N)
    =59.2 N

    I did the same for Box 2 and Box 3, but I multiplied their applied forces times the coefficient of kinetic friction given, 0.64.

    Box 2 = 64 N
    Box 3 = 76.8 N

    Can anyone offer any insight into what I did wrong, or if I did this correctly? Thank you.
    Last edited: Oct 30, 2009
  2. jcsd
  3. Oct 30, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor



    Static friction ≤ μsRN
    Dynamic friction = μkRN
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