A rocket creates thrust through a controlled explosion caused by a violent chemical reaction between the fuel and oxidizer. The expanding gases from the explosion are blasted out of the rocket's backside via a nozzle. The nozzle is a specifically designed exhaust that funnels the hot, high-pressure gas produced by combustion into a stream that departs at hypersonic speeds, more than five times the speed of sound, from the back of the nozzle.

According to Isaac Newton's third law of motion (opens in new tab), every action has an equal and opposite reaction, therefore the 'action' force that propels the rocket forward must be balanced by an equal and opposite force that propels the rocket forward. This force operates specifically on the upper wall of the combustion chamber, but because the rocket motor is intrinsic to each rocket stage, we can consider it acting on the rocket as a whole.