Repeat Problem 3-13 for a 24,000-lb truck with a cross-sectional area of 100 ft2 and coefficient of drag of 0.5 traveling at 50 mi/hr.
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):
Ra = 0.5 × [(2.15pCDAu2) / g]
Ra = 0.5 × [(2.15(0.0766)(0.5)(100)(50)2 / 32.2]
Ra = 319.7 lb
The rolling resistance is (using Equation 3.15):
Rr = (Ca + 1.47Cbu) × W
Rr = (0.02445 + 1.47(0.00044)(50)) × 24,000
Rr = 1363 lb
The grade resistance is:
RG = WG
RG = (24,000)(0.05)
RG = 1200 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(319.7 + 1361 + 1200)(50)) / 550]
P = 385 hp
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What will be an ideal response?