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