A 18 lb cube rests on the frictionless cylindrical surface A

A 1-8 lb cube rests on the frictionless cylindrical surface. An elastic cord having a stiffness of k = 2.2 lb/ft is attached to the cube at C and to the base of cylinder at point A. If the block is released from rest at B, find the unstretched length of the cord so that the cube begins to leave the surface at the instant theta = 41.5 degree. Treat the cube as a particle.