The assembly weighs $10 \mathrm{lb}$ and has a radius of gyration $k_{G}=0.6 \mathrm{ft}$ about its center of mass $G .$ The kinetic energy of the assembly is $31 \mathrm{ft} \cdot \mathrm{lb}$ when it is in the position shown. If it rolls counterclockwise on the surface without slipping, determine its linear momentum at this instant.