# Angular displacement & velocity

1. Jul 30, 2009

### pttest

1. The problem statement, all variables and given/known data
After fixing a flat tire on a bicycle you give the wheel a spin. Its initial angular speed was 7.05 rad/s and it rotated 13.7 revolutions before coming to rest. What was its average angular acceleration?

2. Relevant equations
ω2 = 2 alpha . theta

3. The attempt at a solution
ωf = 0
theta = 13.7 rev = 86.04 rad
alpha = ?

Using the above equation : alpha = -0.327rad/s^2. But the answer is wrong. Could someone help me??

2. Jul 30, 2009

### slider142

EDIT: Ignore this post. The poster below is correct.

Last edited: Jul 30, 2009
3. Jul 30, 2009

### kuruman

Angular acceleration is change in angular velocity divided by the time interval over which this change takes place. The dimensions don't come out right otherwise.

Your numerical answer seems to be correct. what makes you think it is not?

4. Jul 31, 2009

### pttest

When I enter that answer in the box (masteringphysics HW), it says try again. So thought it might be a wrong answer.

5. Jul 31, 2009

### kuruman

In my opinion, "Mastering Physics" sometimes behaves incredibly stupidly and by this I mean it asks ambiguous questions. Try entering your answer without the negative sign.