1. Sep 10, 2009

### LBloom

1. The problem statement, all variables and given/known data

"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 1400 kg, uses a uniform cylindrical flywheel of diameter 1.90 m and mass 210 kg, and should be able to travel 330 km without needing a flywheel "spinup."

a)Make reasonable assumptions (average frictional retarding force = 500 N, twenty acceleration periods from rest to 93 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.
b)What is the angular velocity of the flywheel when it has a full "energy charge"?
c)About how long would it take a 150-\rm hp motor to give the flywheel a full energy charge before a trip?

2. Relevant equations

Kfw=1/2*I*$$\omega$$^2
I=1/2*Mfw*R^2
Frictional Work: W=Ffd

3. The attempt at a solution

I converted the km/h to m/s, to get that the car maxes out at (155/6) m/s
I converted km to m, to get that the total trip was 330000m

I found that I=94.7625 and that the Wf=1.65*10^8
I assume that I calculate the acceleration to find the average velocity for the entire trip, although I'm not quite sure how to do that yet, but I'll get started on that. Honestly, I'm just confused what the significance of the fact that there are 20 acceleration periods. Could I divide the trip in 20, and figure out the work required for that distance and multiply it by 20, or is that unneeded?

Edit: I think I found the average velocity of the trip to be 12.9167 m/s. Can anyone tell me if i'm right or on the right track?

Last edited: Sep 10, 2009