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Homework Help: Find the Power to move a car on an incline

  1. Jan 25, 2012 #1
    1. The problem statement, all variables and given/known data
    What power is needed to move a 3000lb car up an 8 degree incline with a constant speed of 50mi/h against a frictionless force of 80lb?

    ans. 68hp

    2. Relevant equations

    [itex]P = Fv[/itex]

    1hp = 550 lb-ft/s

    3. The attempt at a solution
    1st i draw free body diagram
    summation of force in the horizontal direction
    Fx = 0 = 3000sin(8) + 80 - Fn
    Fn = 497.52lb

    Power = velocity x force
    = (50mi/h)(1/3600s)(1ft/0.3048m)(1000m/1km)(1km/0.62137mi)
    = 36,485.56 lb-ft/s
    = (36,485.56 lb-ft/s)(1hp/(550lb-ft/s))
    = 66.34 horsepower
    where did i get wronged?
  2. jcsd
  3. Jan 25, 2012 #2


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    Homework Helper

    I get the same answer you have.
  4. Jan 25, 2012 #3
    I think your answer is correct. I get the same result either by figuring the forces and multiplying by velocity (as you did) or by using the 80 pound force times velocity added to the change in potential energy per second.
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