Definition
A principle of physics stating that for every action there is an equal and opposite reaction. In aviation, this law explains how thrust is produced: when a propeller or jet engine pushes a mass of air rearward, the air pushes the aircraft forward with equal force. It also underlies lift production, since wings deflect air downward and the air pushes the wing upward in return.
Plain English
If you push on something, it pushes back on you with the same force in the opposite direction. An aircraft moves forward because its engine throws air backward, and the air pushes the aircraft forward in return.
Context Anchor
Encountered in aerodynamics discussions, especially when explaining lift, thrust, propeller effects, and aircraft control forces.
Derivation
Named after Sir Isaac Newton, the English scientist who published these laws of motion in 1687. 'Reaction' comes from the Latin re- ('back') and agere ('to act') -- literally 'to act back.' That captures the idea well: every push produces a push back.
Why Pilots Care
Understanding this law helps pilots see why thrust, lift, and even control surface effects work the way they do. It's the basis for how propellers pull the airplane forward and how wings produce lift by deflecting air.
Analogy
Step off a small boat onto a dock and the boat slides backward as you step forward. Your push on the boat sends it one way; its push on you sends you the other.
Grounding Statement
Picture a wing sending air downward; at the same time, the air pushes the wing upward.
Intuition Check
"Reaction" does not mean a delayed response here. It means an equal force that happens at the same time in the opposite direction.
Example Sentence 1
By Newton's Third Law, the propeller pushes air rearward and the air pushes the aircraft forward, producing thrust.
Example Sentence 2
A wing deflects air downward, and the upward reaction force becomes lift that keeps the airplane flying.