Definition
A law of motion stating that for every action there is an equal and opposite reaction. When one object exerts a force on a second object, the second object simultaneously exerts a force of equal magnitude in the opposite direction on the first.
Plain English
Whenever something pushes on another thing, that thing pushes back just as hard in the opposite direction. Forces always come in matched pairs.
Context Anchor
Seen when explaining how wings and propellers make an airplane move by pushing air.
Derivation
Named for Sir Isaac Newton, the English physicist who published these laws of motion in 1687. It is called the 'third' law simply because it is the third in his original sequence of three.
Why Pilots Care
It explains how a propeller or jet pushes air backward to create forward thrust, and how a wing produces lift by deflecting air downward.
Analogy
If you push on a wall, the wall pushes back on your hand with the same strength in the opposite direction. The airplane example is harder to see because the object being pushed is air, but the same rule applies.
Grounding Statement
Step off a small boat onto a dock: the boat slides backward as you step forward. You pushed the boat back; the boat pushed you forward. Same force, opposite directions, at the same instant.
Intuition Check
The two forces do not cancel each other on the same object. They act on two different objects: one force is on the air, and the other force is on the airplane.
Example Sentence 1
Newton's Third Law explains why a propeller produces thrust: it accelerates a mass of air rearward, and that air pushes the aircraft forward with equal force.
Example Sentence 2
When the wing pushes air downward, the air pushes the wing upward, generating lift through Newton's Third Law.