You are holding a bulletin board weighing 9.03 N in place against a wall while your friend secures it to the wall. To keep it from slipping, you apply a force perpendicular to the bulletin board, pressing it directly into the wall. How large must this force be if the coefficient of static friction with the wall is 0.39

Air is to be preheated by hot exhaust gases in a cross-flow heat exchanger before it enters the furnace. air enters the heat exchanger at 95 kpa and 20°c at a rate of 0.6 m^3/s. the combustion gases (cp = 1.10 kj/kg°c) enter at 160°c at a rate of 0.95 kg/s and leave at 95°c. determine the rate of heat transfer to the air and its outlet temperature.