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Homework Help: Angular acceleration of a fan

  1. Nov 4, 2011 #1
    angular acceleration of a fan!!!!!!!!!!

    1. The problem statement, all variables and given/known data

    An electric fan is turned off, and its angular velocity decreases uniformly from 500 rev/min to 200 rm rev/min in 4.00 s.
    Find the angular acceleration in rev/s and the number of revolutions made by the motor in the 4.00 interval.

    2. Relevant equations



    3. The attempt at a solution



    I got -1.25 for alpha in rev/sec^2, which is correct.

    How come the number of revolutions isn't 5 revolutions?
    (alpha) (time)=
    (-1.25)(4 sec)=5 rev (-5) rev
     
  2. jcsd
  3. Nov 4, 2011 #2

    gneill

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    Staff: Mentor

    Re: angular acceleration of a fan!!!!!!!!!!

    Acceleration x time = velocity, not distance!
     
  4. Nov 4, 2011 #3
    Re: angular acceleration of a fan!!!!!!!!!!

    ohhhhhhhhhhh........ ok thanks. hmmm....... i'm not sure what formula i should use. i was going to try x=xo+vot+1/2at^2 but i can;t. ..................
     
  5. Nov 4, 2011 #4

    gneill

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    Staff: Mentor

    Re: angular acceleration of a fan!!!!!!!!!!

    Why not? (assuming of course that you'd use the angular form of the equation)
     
  6. Nov 4, 2011 #5
    Re: angular acceleration of a fan!!!!!!!!!!

    i dont have xo right?
    we didn't cover this yet.......
     
  7. Nov 4, 2011 #6

    gneill

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    Staff: Mentor

    Re: angular acceleration of a fan!!!!!!!!!!

    Assume xo is zero. The count of revolutions (distance) begins, and the clock starts ticking, at the instant the fan is turned off.
     
  8. Nov 4, 2011 #7
    Re: angular acceleration of a fan!!!!!!!!!!

    okay so :

    0+(-1.25 rev/s^2)(4 sec)+ 1/2(-1.25 rev/s^2)(4^2)
    x=-5.00+ -12.50
    x=-17.50

    how do i make this angular?
     
  9. Nov 4, 2011 #8

    gneill

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    Staff: Mentor

    Re: angular acceleration of a fan!!!!!!!!!!

    You're using angular values, so it's angular.

    But you've got a problem. You're using an acceleration where the initial angular velocity is required.
     
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