Angular displacement & velocity

[pttest]

Homework Statement

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?

Homework Equations

ω2 = 2 alpha . theta

The Attempt at a Solution

ωi = 7.5 rad/s
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??

Thanks in advance

 

[slider142]

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

 

[kuruman]

The "average angular acceleration" has a simple definition: the total change in angular velocity over the total angle covered (in radians), both of which are given by the problem. This is completely analogous to "average linear acceleration", as are most of the definitions of angular quantities. This gives quite a different answer to yours, which is part of an equation of instantaneous quantities (derivatives). Since it is quadratic in nature, it would need corrections to the linear approximation you are making with average quantities.

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?

 

[pttest]

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

 

[kuruman]

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

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.