# Homework Help: Angular Acceleration Problem Why am I not right?

1. Oct 12, 2009

### tjohn101

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.

2. Oct 12, 2009

### Staff: Mentor

You're off by a factor of ten.

3. Oct 12, 2009

### 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?

4. Oct 12, 2009

### Staff: Mentor

Perfect!

5. Oct 12, 2009

### 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?

6. Oct 12, 2009

### Staff: Mentor

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.

7. Oct 12, 2009

### tjohn101

Now there is one small problem. The answer is wrong. Would it be positive?

8. Oct 12, 2009

### Staff: Mentor

The question is ambiguous as to sign, since no direction or sign convention was given. I would just give the magnitude of the acceleration.