F=MA.......... Basic Physics.
Force (Thrust) =
Mass (of the Air) X
Acceleration (of the Air).
Thrust (Force) is what we need for sustained flight. As mentioned earlier, the Mass of air "in" is the same as the Mass of air "out", and is thus a constant (ignoring the negligible addition of mass due to fuel added). Thus, if we want Thrust, we must
Accelerate the air to a higher speed than that when entering the engine.
Regards,
Old Smokey