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Angular Velocity and Kinetic Energy Problem?

  1. Mar 21, 2008 #1
    A wheel turns with a moment of intertia of 16.0kg m^2 and has 10m N applied to it. if it starts from rest calculate the angular velocity. Also what is the kinetic energy 10seconds later?

    The wheels angular velocity i used T=IA, and solved for Angular velocity solving for a i got .625rad/sec^2 . For knetic energy .625rad/sec^2 i get to 6.25rad/sec. then i take 1/2IW^2, and i get 312 J

    Is my work correct ? If i made a mistake , can you please tell me where ?
     
  2. jcsd
  3. Mar 21, 2008 #2
    Hi,:smile:

    If I'm not wrong (which I am most of the time), you need more data (radius of wheel and distance from the centre where the 10mN force is applied) to solve this problem :confused:
     
  4. Mar 21, 2008 #3

    Doc Al

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    Staff: Mentor

    It looks OK to me, assuming you meant to find the angular acceleration (not the angular velocity). To find the angular velocity, you'd need to know at what time.

    10 m N (better: 10 N-m) is a torque, not a force.
     
  5. Mar 21, 2008 #4
    i meant the angular velocity, i wanted to find, how would i find it ?
     
  6. Mar 21, 2008 #5

    Doc Al

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    Staff: Mentor

    Depends on when. You found the angular velocity after 10 s, which you needed to calculate the KE.

    Did you present the problem exactly as it was given to you?
     
  7. Mar 21, 2008 #6
    Yes Doc, its correct. THe problem is copied right
     
  8. Mar 21, 2008 #7
    OK, I thought it was 10 milli Newton force.

    May I suggest using [tex]\omega=\omega_o+\alpha t[/tex]
     
  9. Mar 21, 2008 #8
    and KE =[tex]\frac{I{\omega}^2}{2}[/tex]
     
  10. Mar 21, 2008 #9

    Doc Al

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    Staff: Mentor

    This is slightly ambiguous. Do they mean: 10 seconds later, calculate the angular velocity and the kinetic energy. (If so, then you did calculate the angular velocity.)

    In order to calculate the velocity and KE, you must first calculate the angular acceleration. Which you did, even though you incorrectly called it angular velocity.

    You never know! :wink: That's why I asked if the problem was presented exactly as given.
     
  11. Mar 21, 2008 #10
    For angular velocity, i dont know the time, the time is implied toward the 2nd part of question
     
  12. Mar 21, 2008 #11
    Yeah. The question's language is a bit confusing. But I feel like they want angular velocity at 10 seconds, though they don't mention it.

    Why not give them a general answer
    [tex]\omega=0.625t[/tex]

    They can plug in whatever t they want and get the result :wink:
     
  13. Mar 21, 2008 #12
    Now you see the torture i go through to understand problems lol. All his questions are worded badly, must be cause he is from another Country heh
     
  14. Mar 21, 2008 #13
    Thats correct mate.
     
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