PIZZA PIZZA
A new pizza store is planning to open. It will, of course,
offer free delivery, and therefore need a small delivery car
with a large sign attached. The sign (a flat plate) is 1.5 ft high
and 5 ft long. The boss (having no feel for fluid
mounts the sign bluntly facing the wind. One of his drivers
is taking fluid mechanics and tells his boss he can save lots
of money by mounting the sign parallel to the wind. (See
Fig. .) (a) Calculate the drag (in lbf) on the sign alone at
40 mi/h (58.7 ft/s) in both orientations. (b) Suppose the car
without any sign has a drag coefficient of 0.4 and a frontal
area of 40 ft2. For V =40 mi/h, calculate the total drag of the
car–sign combination for both orientations. (c) If the car has
a rolling resistance of 40 lbf at 40 mi/h, calculate the horsepower
required by the engine to drive the car at 40 mi/h in
both orientations. (d) Finally, if the engine can deliver 10 hp
for 1 h on a gallon of gasoline, calculate the fuel efficiency
in mi/gal for both orientations at 40 mi/h.
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