A 0.5-kg block B slides without friction inside a slot cut in arm OA which rotates in a vertical plane. The rod has a constant angular acceleration 2 = 10 rad/s2. Knowing that when 0 = 45° and r = 0.8 m the velocity of the block is zero, determine at this instant: a) The force exerted on the block by the arm b) The relative acceleration of the block with respect to the arm B A