A portion of the flue for a furnace is attached to the ceiling at A. While supporting the free end of the flue at F, a worker pushes in at E and pulls out at F to align end E with the furnace. Knowing that the 10-lb force at F lies in a plane parallel to the yz plane, determine

(a) The angle α the force at F should form with the horizontal if duct AB is not to tend to rotate about the vertical,

(b) The force-couple system at B equivalent to the given force system when this condition is satisfied.