Introduction
Newton's Third Law states that forces always occur in pairs between interacting bodies. To every action there is an equal and opposite reaction. This law emphasizes that forces are interactions between objects, not isolated entities, and explains how objects influence each other's motion.
Statement of the Law
To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. Mathematically: F_AB = -F_BA, where F_AB is the force exerted by body A on body B, and F_BA is the force exerted by body B on body A.
Characteristics of Action-Reaction Pairs
Action-reaction forces: (1) Have equal magnitude; (2) Have opposite direction; (3) Act on different bodies (cannot cancel each other); (4) Are of the same type (both contact or both field forces); (5) Act simultaneously (neither causes the other). The terms 'action' and 'reaction' are arbitrary - either force could be called the action.
\nIntroduction
Newton's Third Law states that forces always occur in pairs between interacting bodies. To every action there is an equal and opposite reaction. This law emphasizes that forces are interactions between objects, not isolated entities, and explains how objects influence each other's motion.
Statement of the Law
To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. Mathematically: F_AB = -F_BA, where F_AB is the force exerted by body A on body B, and F_BA is the force exerted by body B on body A.
Characteristics of Action-Reaction Pairs
Action-reaction forces: (1) Have equal magnitude; (2) Have opposite direction; (3) Act on different bodies (cannot cancel each other); (4) Are of the same type (both contact or both field forces); (5) Act simultaneously (neither causes the other). The terms 'action' and 'reaction' are arbitrary - either force could be called the action.
\nCommon Examples
Examples: You push on a wall, wall pushes back on you; Earth pulls on you (weight), you pull on Earth with equal force; Rocket exhaust pushes down, exhaust pushes rocket up; Box on table pushes down on table (normal force), table pushes up on box; Walking - you push backward on ground, ground pushes you forward.
Distinguishing from Balanced Forces
Action-reaction pairs act on different objects and cannot be added to find net force on a single object. Balanced forces act on the same object and cancel to give zero net force. When analyzing an object's motion, only consider forces acting ON that object, not forces it exerts on other objects. This distinction is crucial for correct free-body diagrams.
Applications and Implications
Third law explains: recoil of guns (bullet forward, gun backward), swimming (push water back, water pushes you forward), bird flight (push air down, air pushes bird up), car propulsion (tires push road back, road pushes car forward). In isolation, internal forces cancel by third law, so only external forces change a system's total momentum.
\nSolved Example: Book on Table
A 2 kg book rests on a table. Identify all action-reaction pairs and calculate forces. Solution: Forces on book: Weight W = mg = 2 × 9.8 = 19.6 N downward, Normal force N = 19.6 N upward. Action-reaction pairs: (1) Earth pulls book down (19.6 N) → book pulls Earth up (19.6 N); (2) Book pushes table down (19.6 N) → table pushes book up (19.6 N). Note: N and W are NOT action-reaction; they both act on book and cancel to give equilibrium, but are different types (contact vs field) and could be unbalanced during acceleration.
\n