The $30-\mathrm{kg}$ gear is subjected to a force of $P=(20 t) \mathrm{N}$, where $t$ is in seconds. Determine the angular velocity of the gear at $t=4 \mathrm{~s}$, starting from rest. Gear rack $B$ is fixed to the horizontal plane, and the gear's radius of gyration about its mass center $O$ is $k_{o}=125 \mathrm{~mm} .$