Answer
$ -1.25\ rad/s^2$
Work Step by Step
Given:
initial angular velocity of fly wheel $\omega_o=25\ rad/s$
time taken to stop is $t =20\ s$
The angular acceleration of the fly wheel is
$\alpha =\frac{\omega-\omega_o}{\Delta t}$
$\alpha =\frac{0-25\ rad/s}{20\ s}$
$\alpha = -1.25\ rad/s^2$
