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Static Friction - starting motion

  1. Oct 1, 2007 #1
    To move a large crate across a rough floor, you push on it with a force at an angle of 21 degrees below the horizontal, as shown in the figure:
    [​IMG]

    Find the force necessary to start the crate moving, given that the mass of the crate is 32kg and the coefficient of static friction between the crate and the floor is 0.50.

    [tex]
    F_x = \mu_s * F_N
    [/tex]

    Here is what I did, and got the wrong answer:
    [tex]
    M_c = 32kg \\*
    [/tex]

    [tex]
    \mu_s = 0.50
    [/tex]

    [tex]
    F_x = \mu_s * F_N
    [/tex]

    So...
    [itex]
    F_x = \mu_s * (M_c) * (a_g)
    [/itex]

    [tex]
    F_x = 0.5*(32kg)*(9.8 m/s^2)
    [/tex]

    [tex]
    F_x = 156.8N
    [/tex]

    So...
    [tex]
    Cos(21) = (156.8 N)/C
    [/tex]

    C = 167.96 N

    That answer is wrong... so what am I doing wrong?
     
  2. jcsd
  3. Oct 1, 2007 #2
    Re-examine your acceleration in the X direction.
     
  4. Oct 1, 2007 #3
    I'm sorry, I still don't understand...
    are you saying that the normal force is not equal to (32)*(9.8)?
     
  5. Oct 2, 2007 #4

    learningphysics

    User Avatar
    Homework Helper

    First write the equations for [tex]\Sigma{F_x} = ma_x[/tex] and [tex]\Sigma{F_y} = ma_y[/tex].

    This is the most important step. If you have the equations right, then the problem is easily solved.
     
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