Definition
A phrase from Newton's Third Law of Motion stating that whenever one object exerts a force on a second object, the second object exerts a force of the same magnitude back on the first object in the opposite direction. In aviation, this principle explains how propellers, jets, and control surfaces produce thrust and aerodynamic forces by pushing air one way to move the aircraft the other way.
Plain English
If you push something, it pushes back on you with the same strength in the opposite direction. A propeller pushes air backward, and the air pushes the airplane forward by the same amount.
Context Anchor
Seen when the Instrument Flying Handbook explains Newton’s Third Law and how forces such as lift, thrust, and control inputs are produced.
Derivation
From Newton's Third Law (1687). 'Reaction' here comes from the Latin re- ('back') and agere ('to act') -- literally 'to act back.' The 'reaction' is not an emotional response; it's the return push that any force creates.
Why Pilots Care
This principle explains why deflecting air downward over a wing produces upward lift and why a propeller pushing air rearward moves the airplane forward.
Analogy
Push hard against a wall and the wall pushes back on your hand with matching force; neither force exists without the other.
Grounding Statement
Step off a small boat onto a dock: the boat slides backward as you step forward. Your foot pushed the boat back; the boat pushed you forward. Same idea, every time an aircraft moves air.
Intuition Check
“Equal and opposite” does not mean the airplane’s forces simply cancel out. The two forces act on different things, so they can still produce aircraft motion.
Example Sentence 1
The propeller accelerates air rearward, and by equal and opposite reaction, the aircraft is pushed forward.
Example Sentence 2
Lowering the elevator deflects airflow upward, generating an equal and opposite reaction that lowers the tail and raises the nose.