A 4-kg block is suspended from a spring that has a stiffness of k = 600 N/m. The block is drawn downward 50 mm from the equilibrium position and released from rest when t = 0. If the support moves with an impressed displacement of δ = (10 sin 4t) mm, where t is in seconds, determine the equation that describes the vertical motion of the block. Assume positive displacement is downward.