a 1.50 3 103 -kg car starts from rest and accelerates uniformly to 18.0 m/s in 12.0 s. assume that air resistance remains constant at 4.00 3 102 n during this time. find (a) the average power developed by the engine and (b) the instantaneous power output of the engine at t 5 12.0 s, just before the car stops accelerating'