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Homework Help: Angular Acceleration!

  1. Oct 31, 2006 #1
    Question: A cooling fan is turned off when it is running at 850 rev/min. It turns 1500 revolutions before it comes to a stop.
    A. What is the fan's angular acceleration?
    B. How long does it take for the fan to come to a stop?

    I am pretty stumped with this question. I know that I have to use the equation a= omega (angular velocity)/ time ........and angular velocity= Change in theta/time ....but i am not sure how to go about the problem .......can anyone help?
  2. jcsd
  3. Oct 31, 2006 #2
    You said it: [tex] \alpha = \frac{\Delta \omega}{\Delta t}[/tex]
    Be careful though. 850rev/min is the frequency, NOT the angular velocity. But these are related by [tex] 2\pi f = \omega[/tex].
  4. Oct 31, 2006 #3
    thanks Euclid ..... so I got an angular velocity of 5340 rev/sec .......but how would I figure out the time since a= change in angular velocity/time
  5. Oct 31, 2006 #4
    i am really confused.... i dont even know if i got the angular velocity right ..... 5340 revolutions per second seems so much ....

  6. Oct 31, 2006 #5
    Firstly, angular velocity is in units of 1/s or rad/s, not rev/sec.

    Secondly, sorry for misleading you, I read the question wrong. You want to use the equation:

    [tex] 2 \alpha \Delta \theta = \omega_f^2 - \omega_i^2[/tex]

    How many radians does 1500 revolutions correspond to?
  7. Oct 31, 2006 #6
    would 1500 revolutions = 3000 radians? or am i totally off
  8. Oct 31, 2006 #7
    i know that the acceleration must be negative ..... considering wf=0 .... and -w1^squared= 2Atheta
  9. Oct 31, 2006 #8
    anyone there ?? please help!!