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warfreak131
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Homework Statement
A car moving with speed [itex]v[/itex] is subject to a drag force [itex]F_{d}=0.5\rho A c_{d}v^{2}[/itex], where [itex]\rho[/itex] is the density of air ([itex]1.25 kg/m^{3}[/itex] at STP), A is the cross sectional area of the car, and [itex]c_{d}[/itex] is the drag coefficient.
A Honda Civic has [itex]A=1.9 m^{2}[/itex] and [itex]c_{d}=0.36[/itex].
Assuming gasoline contains 125 MJ/gallon of energy, and that the car's engine is 25% efficient, calculate the gasoline consumption (in miles per gallon) needed to overcome this drag force if the car is traveling 75 mi/h (=33.3 m/s). Hint, 1 mile = 1600 m.
B. What is the power output of the engine needed to overcome this drag force at 33.3 m/s? Hint, 1 horsepower = 750 watts.
Homework Equations
W=Fd
P=W/t=Fv
The Attempt at a Solution
From the given information, I know that the drag force is equal to 474 N (using MKS units), and the power needed to overcome the drag is approximately 15,800 W.
From unit analysis, I have to somehow convert MJ/gal into mi/gal, which would mean multiplying by mi/MJ, but I am not sure how to get that.
I know from the efficiency of the engine, that you can only extract a maximum of 31 MJ/gal of gas.