Here's why:
* Newton's Third Law: For every action, there is an equal and opposite reaction. This means the forces always act on *different* objects.
* Propulsion Force: This is the force generated by the airplane's engines, pushing it forward. The reaction to this force is the exhaust gases being pushed backward.
* Gravity Force: This is the force exerted by the Earth on the airplane, pulling it downwards. The reaction to this force is the airplane's gravitational pull on the Earth (although negligible due to the mass difference).
The key point is that the forces of propulsion and gravity act on the *same* object (the airplane). They are not acting on different objects as required by Newton's Third Law.
Therefore, they are not an action-reaction pair.
Instead, think of it like this:
* Action: The engines push air backward (propulsion)
* Reaction: The air pushes the airplane forward.
* Action: The Earth pulls the airplane downwards (gravity)
* Reaction: The airplane pulls the Earth upwards (negligibly small).
