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Angular acceleration of a fan

  • Thread starter valeriex0x
  • Start date
  • #1
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angular acceleration of a fan!!!!!!!!!!

Homework Statement



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.

Homework Equations





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
 

Answers and Replies

  • #2
gneill
Mentor
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Acceleration x time = velocity, not distance!
 
  • #3
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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. ..................
 
  • #4
gneill
Mentor
20,792
2,770


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. ..................
Why not? (assuming of course that you'd use the angular form of the equation)
 
  • #5
43
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i dont have xo right?
we didn't cover this yet.......
 
  • #6
gneill
Mentor
20,792
2,770


i dont have xo right?
we didn't cover this yet.......
Assume xo is zero. The count of revolutions (distance) begins, and the clock starts ticking, at the instant the fan is turned off.
 
  • #7
43
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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?
 
  • #8
gneill
Mentor
20,792
2,770


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?
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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