Newton Third Law
Every action has an equal and opposite reaction.
Formula
Variables
Example
Rockets push exhaust down
How it works
Newton's Third Law says forces always come in pairs: if object A pushes on object B, then object B pushes back on object A with the same size force in the opposite direction. The two forces act on different objects, which is why they do not cancel each other on one object. This happens because interactions are mutual: a push, pull, collision, or contact force always affects both sides of the interaction.
When to use it
Use this law whenever two objects interact and you want to identify the paired forces, such as walking, pushing a wall, or a ball hitting a bat. It is especially useful in mechanics problems where you need to match forces on each object separately.
Worked example, step by step
Example: A person pushes horizontally on a box with a force of 120 N to the right. Step 1: Identify the action force on the box: F_AB = 120 N right. Step 2: Apply Newton's Third Law: the box exerts an equal and opposite force back on the person. Step 3: Write the reaction force: F_BA = -120 N, or 120 N to the left. Final answer: The force pair is 120 N right on the box and 120 N left on the person.
Common mistakes to avoid
- thinking the two forces cancel because they are equal, even though they act on different objects
- mixing up which object is A and which is B when naming the force pair
- assuming the reaction force happens later instead of at the same time as the action force
Did You Know?
A rocket flies by throwing gas backward; the equal and opposite reaction pushes it forward, even in empty space.
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More in Physics Laws & Theorems
View allNewton's First Law (Inertia)
BasicAn object stays at rest or in uniform motion unless acted on by a net force.
Newton's Second Law
BasicNet force equals mass times acceleration (rate of change of momentum).
Newton's Third Law
BasicFor every action there is an equal and opposite reaction.
Conservation of Momentum
IntermediateTotal momentum of an isolated system stays constant.
Conservation of Energy
BasicEnergy cannot be created or destroyed, only transformed.
Archimedes' Principle
IntermediateBuoyant force equals the weight of fluid displaced.