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Homework Help: Friction & Acceleration

  1. Apr 17, 2009 #1
    A 200kg crate us pushed horizontally with a force of 700 N. If the coefficient of friction is 0.20, calculate the acceleratio of the crate.

    I tried doing this but I don't seem to be getting the right answer.

    m = 200 kg
    f(app)= 700 N
    U = 0.20
    Ff = (0.20)(200)(-9.81)
    =-392.4 N
    Fnet = 700-(-392.4)
    = 1092.4 N
    Fnet = mass x acceleration
    1092.4 = 200 (a)
    5.462 m/s^2 = a

    The answer booklet says that I should be getting 3.3m/s^2 as my answer.
  2. jcsd
  3. Apr 17, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your error is giving g a minus sign. g is just the magnitude of the acceleration due to gravity.

    So friction = μN = μmg. That's just the magnitude of the friction force. It's up to you to assign it the proper direction and sign; since it opposes the motion it will be negative (assuming the crate moves in the positive direction).

    Set ΣF = ma and solve for a.

    (In a nutshell: The friction reduces the effect of the applied force.)
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