A $150-$mm$-$diameter shaft with a mass of $20$ kg is rotating at $900$ rpm$.$ A pulley mounted on the shaft has a mass moment of inertia of $0\mathrm{.}15$ kg $$ m$2.$ If the shaft and the pulley coast to a stop due to a tangential frictional force of $8$ lb at the outer radius of the shaft, determine the time required.