Determine the horsepower developed by a passenger car traveling at a speed of 50 mi/h on an upgrade of 5% with a smooth pavement. The weight of the car is 3500 lb and the cross-sectional area of the car is 40 ft2.
What will be an ideal response?
First, determine all of the resistive forces, air, rolling, and grade, acting on the
vehicle, and then determine its horsepower requirement.
The air resistance is (using Equation 3.13):
The rolling resistance is (using Equation 3.14):
Rr = (Crs + 2.15Crvu2) × W
Rr = (0.012 + 2.15(0.65 × 10–6)(50)2) × 3500
Rr = 54.2 lb
The grade resistance is:
RG = WG
RG = (3500)(0.05)
RG = 175.0 lb
Since these are the only forces acting on the vehicle, one can now determine the
horsepower requirement. Use Equation 3.17.
P = 1.47Ru / 550
P = [(1.47(102.3 + 54.2 + 175.0)(50)) / 550]
P = 44.3 hp
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