peripheral velocity

Problem 140
At what angular velocity will the stress of the rotating steel ring equal 150 MPa if its mean radius is 220 mm? The density of steel 7.85 Mg/m³.
 

Solution 140
$CF = M \omega^2 \bar x$
 

Thus,
$CF = \rho A \pi R \omega^2 (2R / \pi)$