An 8-lb weight stretches the spring 1 ft on coming to rest at equilibrium. The whole 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. Compute the equation of the motion. Beyond its first return to the equilibrium position, after each period of how many seconds does the mass return to the equilibrium position?