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What is Thrust? As a plane's propellers spin, jet engines thrust, or rocket engines burn, they all performing the same basic task: thrusting particles of some mass backwards. In doing so, they cause the plane to which they are attached to move forwards. 
Thrust as a Reaction If you stand on a rolling object, a skateboard, for example, and step off, you're demonstrating one of Newton's fundamental laws of physics, the Third Law of Motion. As you step off that skateboard, what you're actually doing is exerting a force on it, a force backwards. Your body never realizes it, because you never really think about it, but if you consciously notice what you're doing, you're pushing on the skateboard. What actually causes you to move is the fact that the skateboard is pushing back in exactly the opposite direction (forwards on your body) with exactly the same amount of force, as figure 1.4.2 shows. 
Another one of Newton's laws states that the sum of all the forces on an object is equal to the mass of the object multiplied by the object's acceleration. Since your body is much more massive than the skateboard, the skateboard gains a much higher acceleration from the force of your body, and your body accelerates more slightly. This is why you don't fly off the skateboard, and why the skateboard starts to roll away from you rapidly. Likewise, a plane's propulsion system pushes against other objects to cause the plane to move. A propeller or jet engine forces air backwards at high speed. Although the air has a low mass, this is offset by the high speed at which it is traveling. Similarly, a rocket engine forces fuel backwards, thrusting spent fuel particles backwards. Just like the other types of propulsion systems, the fuel particles push back against the plane, forcing it forwards. The force from the propulsion system causes the plane to accelerate forwards, counteracting the drag of the plane, keeping it in motion. 
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