# THe change of energy HELP please!

1. Nov 8, 2009

1. The problem statement, all variables and given/known data
A flywheel is a solid disk that rotates about an axis that is perpendicular to the disk and its center. Rotating flywheels provide a means for storing energy in the form of rotational kinetic energy and are being considered as a possible alternative to batteries in electric cars. The gasoline burned in a 298-mile trip in a typical midsize car produces about 1.1 x109 J of energy. How fast would a 13-kg flywheel with a radius of 0.34 m have to rotate in order to store this much energy? Give your answer in rev/min.

2. Relevant equations

K= 1/2 I w2

3. The attempt at a solution

1.1 x 109= .5 (13 (.34)2) w2
solved for w n got the wrong answer. Please tell me where im going wrong!!

2. Nov 8, 2009

### hage567

Are you expressing your answer in rev/min? Are you sure you are using the right formula for the moment of inertia of a solid disk?

3. Nov 8, 2009

### flatmaster

You used the wrong moment of inertia I Find the moment for a SOLID disc

4. Nov 8, 2009

Ok
so i found the moment of inertia of a solid to be 2/5mr2. Used that in the eqn to solve for angular velocity. Then I multiplied that number by 2pie n divided by 60 to get 6330.554 rev/min. This is wrong too! :/

5. Nov 8, 2009

### hage567

That is the one for a solid sphere, you want the one for a solid disk (or cylinder).

6. Nov 8, 2009