# Angular Acceleration Problem.. Why am I not right?

by tjohn101
Tags: acceleration, angular
 P: 93 1. The problem statement, all variables and given/known data The blades in a blender rotate at a rate of 7900 rpm. When the motor is turned off during operation, the blades slow to rest in 4.0 s. What is the angular acceleration as the blades slow down? Vi= 7900 rpm = 82.72860654 rad/sec (This may be my error) Vf= 0 t= 4 secs 2. Relevant equations $$v = v_0 + a t$$ 3. The attempt at a solution I've tried solving for a in the answer above but the answer is incorrect.. I did: 0 = 82.72860654 + a(4) and when solved got an answer of -20.68215164. I'm unsure of why this is NOT correct. Any help is greatly appreciated. I think the problem may be in my conversions to rad/sec or maybe I am forgetting to convert something.
Mentor
P: 41,309
 Quote by tjohn101 Vi= 7900 rpm = 82.72860654 rad/sec (This may be my error)
You're off by a factor of ten.
P: 93
 Quote by Doc Al You're off by a factor of ten.
Please tell me if this is the right method:

(7900*2pi)/60

If I do this then I get 827.2860654. Does that sound correct?

Mentor
P: 41,309
Angular Acceleration Problem.. Why am I not right?

 Quote by tjohn101 Please tell me if this is the right method: (7900*2pi)/60 If I do this then I get 827.2860654. Does that sound correct?
Perfect!
P: 93
 Quote by Doc Al Perfect!
Okay, and when I solve using the equation above I get the acceleration as being -206.8215164. It is already in rad/sec^2, correct? No conversions needed there?
Mentor
P: 41,309
 Quote by tjohn101 Okay, and when I solve using the equation above I get the acceleration as being -206.8215164. It is already in rad/sec^2, correct? No conversions needed there?
Correct. You are computing Δω/Δt, so the units take care of themselves (since you already converted everything to standard units): (rad/s)/(s) = rad/s^2.
P: 93
 Quote by Doc Al Correct. You are computing Δω/Δt, so the units take care of themselves (since you already converted everything to standard units): (rad/s)/(s) = rad/s^2.
Now there is one small problem. The answer is wrong. Would it be positive?
Mentor
P: 41,309
 Quote by tjohn101 Now there is one small problem. The answer is wrong. Would it be positive?
The question is ambiguous as to sign, since no direction or sign convention was given. I would just give the magnitude of the acceleration.

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