As an automotive engineer, you’re charged with improving the fuel economy of you
ID: 1471185 • Letter: A
Question
As an automotive engineer, you’re charged with improving the fuel economy of your company’s vehicles. You realize that the rotational kinetic energy of a car’s wheels is a significant factor in fuel consumption, and you set out to lower it. For a typical car, the wheels’ rotational energy is 39 %of their translational kinetic energy. You propose a redesigned wheel with the same radius but 20 %lower rotational inertia and 26 % less mass.
What do you report for the decrease in the wheel’s total kinetic energy at a given speed? (express as a percent)
Explanation / Answer
Initially ,
K.E translational = 1/2 * mv^2
K.E rotational = 1/2 * Iw^2 = 0.39 * 1/2 mv^2
Total = 1.39 k
Now,
I = I - 0.2*I = 0.8I
m = 0.74 m
New Kinetic Energy = 0.74* 1/2 * mv^2
New K.E rotational =0.8 * 1/2 * w^2 = 0.8 * 0.39 * 1/2 mv^2
Total = 0.74 k + 0.312 k
Total = 1.052 k
Decrease in total K.E = (1.39 - 1.052) k = 0.338 k
% Decrease = 0.338/1.39 * 100%
% Decrease = 24.3 %
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