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Homework Help: How long does it take for rotating platform to stop

  1. Nov 24, 2004 #1
    Hi my problem is I have a platform that is rotating and it [tex]\omega_0=8\pi[/tex] and the question is how long does it take it to stop. All my work up to here is correct and I have [tex]\sum\tau=I\alpha[/tex] where my [tex]\sum\tau=3.75Nm[/tex] and [tex]I=1.91kgm^2[/tex] and [tex]\alpha=1.96rad/s^2[/tex] so I wanted to use the [tex]\omega= \omega_0+\alpha t[/tex] equation but i dont know final angular velocity and time is my target variable. The solution manual used [tex]\omega=\alpha t[/tex] why is this justified? The [tex]\omega_0[/tex] does not equal zero so why did the solution manual just take it out?
  2. jcsd
  3. Nov 24, 2004 #2


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    Homework Helper

    The final state is when the platform stops, it means does not rotate, is not the final angular velocity zero then??? :rofl:

  4. Nov 24, 2004 #3


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    Science Advisor

    The final [itex]\omega[/itex] is zero. So you have
    [tex]0=\omega_0+\alpha t[/tex]
    [itex]\alpha[/itex] is actually negative (angular speed decreasing), and you know the initial angular speed, so solve the above equation for t and you're done.
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