II. PRINCIPLES OF JET PROPULSION
Jet propulsion is a practical application of Sir Isaac Newton’s third law of motion which states that, ‘for every force
acting on a body there is an opposite and equal reaction’. For aircraft propulsion, the ‘body’ is atmospheric air that is caused
to accelerate as it passes through the engine. The force required to give this acceleration has an equal effect in the opposite
direction acting on the apparatus producing the acceleration. A jet engine produces thrust in a similar way to the
engine/propeller combination. Both propel the aircraft by thrusting a large weight of air backwards (fig. 1), one in the form
of a large air slipstream at comparatively low speed and the other in the form of a jet of gas at very high speed.