Net Work Problem, Rotational Motion,

AI Thread Summary
The discussion focuses on calculating the network done by a ballerina to increase her angular speed from 1.5 rev/s to 4.0 rev/s after drawing in her arms. The initial momentum was incorrectly calculated, and the correct formula for rotational kinetic energy is emphasized as (1/2)Iw^2 rather than Iw^2. Participants highlight the necessity of converting angular speed from revolutions per second to radians per second for accurate calculations. The initial moment of inertia was also questioned, suggesting it should be larger than initially stated. Clarification on the correct approach to compute the work done is sought, indicating a need for further guidance on the calculations.
th3plan
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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 I am correct so multiply by 2pi ?

Thanks Guys
 
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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 I am correct so multiply by 2pi ?

Thanks Guys
 
th3plan said:
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 I am correct so multiply by 2pi ?

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

2. You must convert to rad/s from rev/s.
 
Her initial moment of inertia must have been larger surely?
If I use I\omega = I_2 \omega_2 I get that it is indeed larger.

As a consequence the work you compute is also too large
 
your computation of the inital intertial moment is also wrong
 
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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