The propulsive force developed by a rocket motor during the rocket's powered ascent. The force produced by a rocket motor is described by Newton's second law of motion "Force equals mass times acceleration" and by Newton's third law of motion "for every acting force, there is an equal reacting force in the opposite direction." The amount of forward thrust generated by a motor is defined by the second law (F-MXA), the third law merely says that it must move. In obedience to the second law, the two variables which must be known in predicting a motor's thrust is the mass of exhaust gas expelled at any given moment, and the exhaust's velocity.