Amass weighing 16 pounds stretches a spring 8 3 feet. the mass is initially released from rest from a point 4 feet below the equilibrium position, and the subsequent motion takes place in a medium that offers a damping force that is numerically equal to 1 2 the instantaneous velocity. find the equation of motion x(t) if the mass is driven by an external force equal to f(t) = 10 cos 3t. (use g = 32 ft/s2 for the acceleration due to gravity.)