A 2-lb block rests on the smooth semicylindrical surface. An elastic cord having a stiffne...

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 (θ = 0°), determine the unstretched length of the cord so the block begins to leave the semicylinder at the instant θ = 45°. Neglect the size of the block.