Ocampo,
Well not exactly, the momentum Drag is not due to the air molecules, but due to the forward velocity of the airplane.
The jet engine has to impart additional velocity to air to produce thrust.
When an aircraft is at rest all the velocity that is imparted to the molecules is turned into thrust, but when the aircraft is moving it has to impart additional velocity which has to be in excess of the aircraft velocity to produce thrust.
IF THE VELOCITY IMPARTED TO EXHAUST IS LESS THAN THE AIRCRAFT VELOCITY IT WILL PRODUCE DRAG NOT THRUST AS IN ENGINE FAILURE.
So just the fan is unable to accelerate air at that high velocities because of lack of jet mechanism as in the inner core, hence less thrust at high speeds.
Vicky