# Fan spinning

An electric fan is running on HIGH. After fan has been running of 13.2 minutes, the LOW button is pushed. The fan slows down to 98 rad/sec in 1.83 seconds. The blades of the fan have a radius of 0.62 meters and their deceleration rate is 42.3 rad/sec2.

a) What was the initial angular speed of the fan in rev/sec?

i am pretty sure i have to use the equation:

angular velocity^2=angular velocity^2+2(angular accel)(angular disp)

we have
final velocity= 98
angular accel= -42.3
angular disp= unknown, but we have radius.
time= 1.83

would the angular disp be 2pi?

## Answers and Replies

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Hootenanny
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Science Advisor
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would the angular disp be 2pi?
No. However, can you think of a better formula to use?

perhapes angular veloctiy= initial angular velocity + ang accel*time?

LowlyPion
Homework Helper
perhapes angular veloctiy= initial angular velocity + ang accel*time?
That would be better.

i tried that formula, but my answer wasn't correct...

Hootenanny
Staff Emeritus
Science Advisor
Gold Member
i tried that formula, but my answer wasn't correct...
Perhaps if you detailed your working we could point out where you went wrong.

final velocity= initial velocity+ acceleration*time

98= initial velocity+ (-42.3)(1.83)

175.409= initial velocity?
im pretty sure right off the bat that that is wrong because why would they give you the radius if you didn't need it.... so the radius has to play a part into this.

Hootenanny
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Science Advisor
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final velocity= initial velocity+ acceleration*time

98= initial velocity+ (-42.3)(1.83)

175.409= initial velocity?
Looks good to me
im pretty sure right off the bat that that is wrong because why would they give you the radius if you didn't need it.... so the radius has to play a part into this.
Do you perhaps need the radius for one of the following questions?

nope, its a 1 question problem...i tried that answer and it wasn't correct. so probably need the radius somewhere in there

Hootenanny
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Science Advisor
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nope, its a 1 question problem...i tried that answer and it wasn't correct. so probably need the radius somewhere in there
You're asked for the initial speed in revolutions per second rather than radians per second.

ahhhh theres the problem..ok thanks!