Newton’s Third Law
Newton’s Third Law states that for every action, there is an equal and opposite reaction. This fundamental principle means forces always come in pairs — when one object pushes or pulls another, the second object pushes or pulls back with the same strength but in the opposite direction.
The Law in Simple Terms
Understanding the Law
The key points are:
- Action and reaction forces act on different objects.
- They are equal in magnitude but opposite in direction.
- They happen simultaneously.
Examples in Everyday Life
Example 1: When you jump off a boat, you push the boat backward (action), and the boat pushes you forward (reaction).
Example 2: When you push a wall, the wall pushes back on your hand with equal force.
Example 3: Rockets propel forward by pushing exhaust gases backward at high speed.
Common Misconceptions
- Action and reaction forces do not cancel out because they act on different objects.
- The forces are always present, even if there is no visible motion.
- Equal and opposite forces mean equal force magnitudes, not necessarily equal effects (acceleration depends on mass).
Mathematical Expression
Newton’s Third Law is expressed as:
This means if object A exerts a force on object B, then object B exerts a force of equal magnitude and opposite direction on object A.
Relation to Newton’s Second Law
While Newton’s Second Law (\( \vec{F}_{\text{net}} = m\vec{a} \)) deals with the net force on a single object, the Third Law deals with interactions between two objects. For example, if a car accelerates forward, its tires push backward on the road (action), and the road pushes forward on the tires (reaction), causing acceleration.
Free-Body Diagrams & Forces
- Action-reaction pairs never appear on the same free-body diagram.
- When drawing FBDs, only include forces acting on the object of interest.
- The presence of an equal and opposite force doesn’t mean an object is in equilibrium—acceleration depends on the net force acting on it.
Collision Forces
During a collision, Newton’s Third Law ensures the forces between the two objects are equal in magnitude and opposite in direction—regardless of their masses:
This principle supports the conservation of momentum: internal forces during collisions cancel out, and only external forces can change the system's momentum.
Explore Newton’s Third Law with Formulas
Click a button to view how Newton’s Third Law applies in equations:
Interactive Demo: Action-Reaction Force Pairs
Click the buttons below to see examples of action and reaction force pairs and their descriptions.