>>14048394Well because the air bounces at the sides of the container or channel and air is made of tiny molecules that basically perform like billiard balls so they bounce away. When you shoot air to a wall then it bounces against the atoms of the object which if solid and stationary forces the air to give way by bouncing it away. It is no different from molecules bouncing away from the propeller blade. Instead of having stationary air be pushed by a moving surface you have moving air pushing a stationary surface, it is very much the same exact thing. The total change in momentum is just the airs momentum at the start compared to at the end and the difference gets transferred into the structure which in this case is the plane.
It's easy enough to see this in action with water or air and some kind of object like a straw, you can bend the straw and blow into it and the air comes out in a new angle or pour water from a faucet into a spoon and it splashes into new directions. The molecules come down, bounce which gives a small amount of force to the object they bounced from which is where the plane gets the thrust to move backwards. Or if you want to go even simpler simply take something solid (light enough) and blow air at it, you can make it move but the air also gets deflected in the process. which is the same way the air movement makes the plane move backwards when it hits it and is forced to change directions.
