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Homework Help: Objects in Incline problem

  1. Feb 24, 2004 #1
    OK here's the problem: An extremely light drivable car with a mass of only 9.50 kg was built. Suppose that the wheels of the car are locked, so that the car no longer rolls. If the car is pushed up a 30° slope by an applied for of 80.0 N, the net accel. of the car is 1.64 m/s2. What is the coefficient of kinetic friction between the car and the incline?

    And where's what I've done:
    m = 9.5
    W = 93.195
    a = 1.64
    Fnet = ma = 15.58
    Fapplied = 80
    Ffriction = Fapplied - Fnet = 64.46
    Fperendicular = CosTheta(W) = 80.709 (also Fnormal)
    Fparallel = SinTheta(W) = 46.5975

    Then I have:
    Coeff of friction = Ffriction/Fnormal = 64.46/46.5975 = 1.3833

    But according to the answers my teacher gave me, and logic (that seems a little bit high for the situation), that isn't right. Please show me where I'm going wrong. Thanks.
     
    Last edited: Feb 24, 2004
  2. jcsd
  3. Feb 24, 2004 #2

    jamesrc

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    Science Advisor
    Gold Member

    You forgot the component of the weight along the incline when calculating Fnet.

    [tex] F_{\rm net} = F_{\rm applied} - F_{\rm parallel} - F_{\rm friction} [/tex]

    Try redoing it from there and see how it works out.
     
  4. Feb 24, 2004 #3
    This teacher is the best science teacher I've ever had, but for some reason she failed to mention that the Fparallel was in the Fnet formula on the angle. And I know I didn't just copy it down wrong, because two of my friends (one of whom made a 36 on the math part of his ACT) couldn't do it either. I'll ask her about it when I get back to school, but thanks for the help. I've got another one I need help on, but I'm gonna start a different thread. Thanks again.
     
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