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TTU Dynamics - Energy Analysis in American Units

  1. Nov 23, 2012 #1
    I am having a problem with dynamics this semester. Our professor gives us "FE-like" questions, which are actually more complicated than FE questions. I can get to where I need to apply the kinetic energy equation but my units look really large and I don't know what to do with W' or how I can get opposing head wind velocity from that.

    1. The problem statement, all variables and given/known data

    An automobile, weighing 1500 lbf is being driven at 50 mph utilizing 75 hp when
    it it struck by head wind that drops its speed by 20%. How fast was the wind?
    Ignore all friction but air drag.

    2. Relevant equations

    F = ma
    E = (T+V)
    T = (1/2)mv2


    3. The attempt at a solution

    F = 1500 lbf
    v1 = 50 mph
    W' = 75 hp
    ε = -20%

    F = ma: 1500 = m(32.17)
    m = 46.63 sl

    v2 = v1(1 + ε): v2 = 50(1 - 0.2)
    v2 = 40 mph

    1 = 1.46 fps
    ∴v1 = 73 fps and v2 = 58.4 fps


    T1 = (1/2)mv12: T1 = (1/2)(46.63)(732)
    T1 =

    T2 = (1/2)mv22: T2 = (1/2)(46.63)(58.42)
    T2 =

    The units here don't make sense, since I need to use lbf to get horsepower. Is there a way I can apply force to energy?
     
  2. jcsd
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