An 8-lb weight stretches the spring 1 ft on coming to rest at equilibrium. The overall system is immersed in liquid which provides a damping force that is 2 times the velocity of the mass. Assume the weight is pulled 1 ft below equilibrium and given initial downward velocity of 2 ft/s. Determine the equation of the motion. Beyond its first return to the equilibrium position, after each and every period of how many seconds does the mass return to the equilibrium position?