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Homework Help: Forces and Power

  1. Oct 30, 2007 #1
    1. The problem statement, all variables and given/known data
    (A) If 18.5 HP are required to drive a 2320 kg automobile at 62.0 km/h on a level road, what is the total retarding force due to friction, air resistance, and so on?
    (B) What power is necessary to drive the car at 62.0 km/h up a 15% grade (a hill rising 15.0m vertically in 100m horizontally).

    3. The attempt at a solution
    18.5 HP = 13795 W
    62.0 km/h = 17 m/s
    A) [tex]P = F*v[/tex]
    [tex]P / v = F[/tex]
    [tex]F = ma[/tex]
    [tex]P / v = ma[/tex]
    [tex]a = P / (v * m)[/tex]
    [tex]13795 / (17 * 2320) = 0.3498N[/tex]
    B) [tex]\alpha = arctan(15/100) = 8.53[/tex]
    [tex]\sum F = F_{up} - 0.3498N - w sin(8.530)[/tex]
    [tex]\sum F = F_{up} - 0.3498N - 3375N[/tex]
    [tex]F_{up} = 3375N[/tex]
    [tex]P = F * v[/tex]
    [tex]P = 3375 * 17 = 57375 W = 77 HP[/tex]

    This seems completely wrong to me... I'm not sure if any of it is right even. Any help would be greatly appreciated.
  2. jcsd
  3. Oct 30, 2007 #2


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    Homework Helper

    Your part a) isn't right. You had the right formula, but didn't use it. F = P/v.

    plug in P plug in v. get F.

    Part b), your work is right... you just need to use the correct force from part a).
  4. Oct 30, 2007 #3
    Thanks learningphysics! I had no idea I was so close to the answer, I figured all my work was wrong :(
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