1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted