Angular acceleration of a fan

1. Nov 4, 2011

valeriex0x

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. Nov 4, 2011

Staff: Mentor

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

Acceleration x time = velocity, not distance!

3. Nov 4, 2011

valeriex0x

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. ..................

4. Nov 4, 2011

Staff: Mentor

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

Why not? (assuming of course that you'd use the angular form of the equation)

5. Nov 4, 2011

valeriex0x

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

i dont have xo right?
we didn't cover this yet.......

6. Nov 4, 2011

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.

7. Nov 4, 2011

valeriex0x

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?

8. Nov 4, 2011

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.