A 2-lb block rests on the smooth semicylindrical surface. An elastic cord having a stiffness k = 2 lb/ft is attached to the block at B and to the base of the semicylinder at point C. If the block is released from rest at A ( \theta= 0° ), determine the unstretched length of the cord so that the block begins to leave the semicylinder at the instant \theta = 45° . Neglect the size of the block.