The equation f=−vex(dm/dt) for the thrust on a rocket, can also be applied to an airplane propeller. in fact, there are two contributions to the thrust: one positive and one negative. the positive contribution comes from air pushed backward, away from the propeller (so dm/dt< 0), at a speed vex relative to the propeller. the negative contribution comes from this same quantity of air flowing into the front of the propeller (so dm/dt> 0) at speed v, equal to the speed of the airplane through the air.