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THe change of energy HELP please!

  1. Nov 8, 2009 #1
    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. jcsd
  3. Nov 8, 2009 #2

    hage567

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    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?
     
  4. Nov 8, 2009 #3
    You used the wrong moment of inertia I Find the moment for a SOLID disc
     
  5. Nov 8, 2009 #4
    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! :/
     
  6. Nov 8, 2009 #5

    hage567

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    That is the one for a solid sphere, you want the one for a solid disk (or cylinder).
     
  7. Nov 8, 2009 #6
    Ok so I used 1/2Mr^2 for inertia.. and did the same thing as before and got 5666.367
    still wrong...
    uhhhhhh!
     
  8. Nov 8, 2009 #7

    hage567

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    Based on what you said in post #4, I don't think you are converting to rev/min properly. You should be dividing by 2pi, and multiplying by 60. Look at your units carefully when you are doing this to make sure they work out to what they should be.
     
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