A boy is riding a sled down a snow covered hill. A girl then jumps onto the sled as it is riding down the hill. Briefly explain what will happen to the (i) coefficient of friction when she jumps on and (ii) the force of friction when she jumps on.

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.