How to find angular acceleration

1. Nov 16, 2008

itsmarasilly

1. The problem statement, all variables and given/known data
A propeller slows from 450 rev/min to 200 rev/min in 3.50 s. What is its angular acceleration?

2. Relevant equations
ac = v^2/r
and v = rw

3. The attempt at a solution
I know this is probably a simple problem, but what do the variables in the relevant equations refer to?

2. Nov 16, 2008

asleight

Here are some more relevant equations:

$$\frac{1 rev}{min}=\frac{1 rev}{60 s}=\frac{\pi rad}{30 s}$$, and
$$\vec{\alpha}_{ave}=\frac{\Delta\vec{\omega}}{\Delta t}$$.