One possibility for a low-pollution automobile is for it to use energy stored in
ID: 1520405 • Letter: O
Question
One possibility for a low-pollution automobile is for it to use energy stored in a heavy rotating flywheel. Suppose such a car has a total mass of 1500 kg , uses a uniform cylindrical flywheel of diameter 1.50 m and mass 230 kg , and should be able to travel 350 km without needing a flywheel "spinup."
Make reasonable assumptions (average frictional retarding force = 470 N , twenty acceleration periods from rest to 92 km/h , equal uphill and downhill, and that energy can be put back into the flywheel as the car goes downhill), and estimate what total energy needs to be stored in the flywheel.
E = ____ J
Make reasonable assumptions (average frictional retarding force = 470 N , twenty acceleration periods from rest to 92 km/h , equal uphill and downhill, and that energy can be put back into the flywheel as the car goes downhill), and estimate what total energy needs to be stored in the flywheel.
=_____ rad/s
About how long would it take a 150-hp motor to give the flywheel a full energy charge before a trip?
t = ____ s
Explanation / Answer
First, the energy lost to the drag force is the work done by the drag force.
The car will travel 350 km against a force of 470 N.
The work = Force * distance = 470*350000 = 1.645 x108 Joules
For the twenty acc periods, you are left to assume that the energy used to acc the car is lost when the car brakes (the energy becomes heat in the brakes). So the energy used to acc the car once to 92 km/h is the final KE of the car... (1/2) mv2
First, convert speed to m/s
92 km / hr = 92000m / 3600 s = 25.56 m/s
And KE = (1/2) * 1500 *25.562 = 489815 J
This is for one acc... for 20 you need 489815 * 20 = 0.979 x 108 Joules
Obviously the energy needed for acceleration is tiny compared to the energy lost to drag. You can add the two...
1.575 x 108 + 0.979 x 108 = approx 1.743 x108 J answer to part (a)
(c) There are 746 watts to one hp, so a 150 hp motor puts out 111900 watts or 111900 Joules per second
So
time = 1.7438 x 108Joules / 111900 J / sec = 1557.6 seconds or 25.96 minutes answer to part (c)
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