@ puneeth
Thrust acts in the forward direction. Drag acts in backward direction.
Form drag: Because of the adverse presuure gradient behind the transition point, there is decrease in kinetic energy of lower levels of boundary layer.Whereas the upper level of boundary layer overrun at this point(called seperation point). Because of seperation, Pressure at trailing edge < Pressure at leading edge. This force acting backward is form drag.
Thrust is a force and force=ma. In case of form drag, kinetic energy of lower boundary layer decreases. so acceleration is reduced and therfore thrust gets reduced.