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Homework Help: Net Work Problem, Rotational Motion, need help

  1. Mar 20, 2008 #1
    A ballerian spins initially at 1.5rev/s when arms are extended. She then draws in her arms to her body and her moment of intertia is .88kg m^2 and her angular speed increases to 4.0rev/s. Determine the network she did to increase her angular speed?


    So i first needed to solve for the initial momentum , which i did using equation IW=I2W2, and i got .43kg m 2, then to get the network i used w=(final)IW^2 - (initial) IW^2

    i got 518J, i am not to sure if i did this right. Can anyone please tell me if my math is correct ? Also for angular speed, in the work calculation i would need to get that to rad/s if im correct so multiply by 2pi ?

    Thanks Guys
     
  2. jcsd
  3. Mar 20, 2008 #2
    A ballerian spins initially at 1.5rev/s when arms are extended. She then draws in her arms to her body and her moment of intertia is .88kg m^2 and her angular speed increases to 4.0rev/s. Determine the network she did to increase her angular speed?


    So i first needed to solve for the initial momentum , which i did using equation IW=I2W2, and i got .43kg m 2, then to get the network i used w=(final)IW^2 - (initial) IW^2

    i got 518J, i am not to sure if i did this right. Can anyone please tell me if my math is correct ? Also for angular speed, in the work calculation i would need to get that to rad/s if im correct so multiply by 2pi ?

    Thanks Guys
     
  4. Mar 21, 2008 #3

    Shooting Star

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    Homework Helper

    1. Rotational KE = (1/2)Iw^2, not Iw^2.

    2. You must convert to rad/s from rev/s.
     
  5. Mar 21, 2008 #4
    Her initial moment of inertia must have been larger surely?
    If I use [tex]I\omega = I_2 \omega_2[/tex] I get that it is indeed larger.

    As a consequence the work you compute is also too large
     
  6. Mar 21, 2008 #5
    your computation of the inital intertial moment is also wrong
     
  7. Mar 21, 2008 #6
    ok, yes i understand what i did wrong on intial momentum problem. I should of used 1/2I1W1=1/2I2W2. But, how do i answer the work done?
     
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