Which situations contain unbalanced forces? Check all that apply.

A book rests on a table.
A book falls to the floor.
A car skids to a stop.
A duck floats on a pond.
A balloon hangs in midair.
A foam ball launches from a spring.

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.