How Do You Calculate Angular Acceleration of a Wheel?

[balletgirl]

Homework Statement

A 70cm diameter wheels accelerates uniformly from 160 rpm to 280 rpm in 4.00s.
Determine:
a) it's angular acceleration, and
b) the radial and tangential components of the linear acceleration of a point on the edge of the wheel 2.00s after it has started accelerating

a=?
w2= 280rpm
w1= 160rpm
t = 4.0s
d= .7m

Homework Equations

a = w2-w1 / t

The Attempt at a Solution

I used that equation for part a, with a= 280rpm - 160rpm /4.0s
and got a= 30 , but I don't feel that is right. Is there a better equation I could have used?

 

[JaWiB]

The formula you used is fine, but more accurately:
\alpha = \frac{{\Delta}\omega}{{\Delta}t}
this gives average angular acceleration over the given time interval.

You need to specify units in your answer (and you'll find you want to convert it to more logical units)