Newton's Third Law — Action-Reaction
Overview
For every action force, there is an equal and opposite reaction force. Critical detail: the two forces act on DIFFERENT objects. Example: you push a wall (action) — the wall pushes you back (reaction). These forces are equal in magnitude, opposite in direction, same type of force, but on different objects, so they do NOT cancel.
Action and reaction always come in pairs
Newton's 3rd Law: for every action force, there is an equal and opposite reaction force. These forces always: (1) act on DIFFERENT objects, (2) are equal in magnitude, (3) are opposite in direction, (4) are of the same type (both contact forces, both gravitational, etc.). Examples: Earth pulls you down (gravity on you) — you pull Earth up with the same force. You push backward on the ground while running — ground pushes you forward.
Why pairs DON'T cancel
Students often wonder: if action = −reaction, why do objects accelerate? The answer is that Newton's 3rd Law pairs act on DIFFERENT objects. To apply F = ma to one object, you only include forces acting ON that object — not the reaction force which acts on the other object. When you push a box, the reaction force (box pushing back on you) acts on YOUR body — it doesn't cancel the force on the box.
- ⚠Thinking action-reaction pairs cancel — they act on different objects so they never cancel each other
- ⚠Not specifying which object the force acts ON when describing 3rd Law pairs
- ⚠Confusing normal force + weight (which are on the SAME object) with a 3rd Law pair