# Flywheels and Rotational Motion

1. Nov 29, 2004

### Soaring Crane

One possibility for a low-pollution automobile is for it to use energy stored in a heavy rotating flywheel of mass 240 kg, and should be able to travel 300 km (300,000 m) without needing a flywheel "spinup."

Make reasonable assumptions (avg. frictional retarding force 500 N, 20 acceleration periods from rest to 90 km/h or 25 m/s, equal uphill and downhill --assuming during downhill, energy can be put back into the flywheel), and show that the total energy needed to be stored in fly wheel is about 1.6 x 10^8 J.

I need help to start this proof. Do you use K_i + U_i = K_f + U_f? What do I do from here?

Thanks for any pointers.

Last edited: Nov 29, 2004
2. Nov 30, 2004

### elote

try using K_rotatational also
k_rot = (1/2)IW^2 --> I is inertia, W is omega (angular speed)

3. Nov 30, 2004

### Integral

Staff Emeritus
What happens to the energy stored in the flywheel if you should be come involved in an accident? I have visions of a massive flywheel with a large rotational kinetic energy busting loose from its housing and ripping off down the road destroying car after car, each of which releases a flywheel! :surprised Talk about a chain reaction accident!

Yeah, I know that is a bit extreme, but containment is an issue.

Also spinups will be necessary. Suppose I drive from my home at 100m to spend a week in the mountains at 1000m? A spinup may well be necessary to meet my needs for a week. Now on the way home, I may find myself with more energy to store then the flywheel is designed for.

Just some thoughts, the fact is, for years I have speculated about using a flywheel for automotive energy storage.

4. Nov 30, 2004

### Integral

Staff Emeritus

According to your assumptions the only loss will be that due to friction. If you simply compute the work done by your friction force over 300km, you should have your answer.