A mass weighing 6.4 lb is attached to a spring hanging from the ceiling, thereby causing the spring to stretch 3.2 feet upon coming to rest at equilibrium. At time t=0, an external force of 5cos(4t) lb is applied to the system. The damping constant for the system is 1.2 lb-sec/ft. The object is released six inch below the equilibrium. Find the equation of motion of the mass. Determine the steady-state solution for the system. Solution.