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Homework Help: Mech eng - find top speed

  1. Sep 4, 2009 #1
    1. The problem statement, all variables and given/known data
    How great a speed can the car in sample 1.6 (with 2.64 axle ratio) maintain when going up a continuous 10 percent grade (1-ft rise in 10-ft horizontal travel)?

    without the incline the top speed is 117 mi/hr, giving the wheel 1513 rpm (calculated in the example)
    radius of wheel = 13in
    2. Relevant equations
    at 4000rpm (engine speed): engine output = 160hp

    W = FV

    3. The attempt at a solution
    because of the rise, the road gives a normal force of 398 lbs in the horizontal direction

    first i converted everything to standard units. then i found the force required to push the vehicle 117mph with 160hp:
    F = W/V = 512.9 lb

    then added the additional force due to the incline:
    F1 = 512.9 + 398 = 910.9 lb
    then i solved for the new velocity:
    V = W/F1 = (convert) = 65.9 mph

    however the answer is supposed to be 72 mph. where was the mistake?
  2. jcsd
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