Answer
$\omega_f=10rad/s$
Work Step by Step
The instantaneous angular velocity is the angular velocity at the end of the 5 seconds. This value can be determined with the equation:
$\omega_f=\omega_0+\alpha t$
Since the disc started at rest, $\omega_0=0rad/s$
Substituting the angular acceleration from part (a), we get:
$\omega_f=\alpha t=(2rad/s^2)(5s)=10rad/s$
