Definition
A force exerted equally and in the opposite direction in response to an applied force, as described by Newton's Third Law of Motion. In aviation, reaction is the principle by which propellers, jet engines, and helicopter rotors produce thrust by pushing air or exhaust gases in one direction so the aircraft is pushed in the opposite direction.
Plain English
When something pushes one way, it gets pushed back equally hard the other way. Engines and propellers use this idea to move aircraft forward by throwing air or exhaust backward.
Context Anchor
Seen in basic aerodynamics, propeller thrust, jet thrust, and explanations of how an airplane responds to air and runway contact.
Derivation
From the Latin re- meaning 'back' and actio meaning 'a doing' or 'action.' A reaction is literally an 'acting back' — the response force that comes back when something is pushed.
Why Pilots Care
Every form of aircraft thrust — propeller, jet, or rocket — works because of reaction. Understanding it makes propulsion, torque, and helicopter anti-torque systems easier to grasp.
Analogy
If you push against a wall while standing on a smooth floor, the wall pushes back on you and you move away from it. That push back is the reaction.
Grounding Statement
Step off a small boat onto a dock: the boat moves backward as you step forward. Your push is the action; the boat's movement is the reaction.
Intuition Check
Reaction does not mean an emotional response here. It means a physical force that acts back because another force was applied.
Example Sentence 1
A propeller produces thrust by pushing air rearward; the forward force on the aircraft is the reaction to that push.
Example Sentence 2
Understanding reaction forces helps explain how a turbojet produces thrust.