Newton's Third Law
Simple Explanation
Newton's third law says that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first. These action-reaction forces always act on two DIFFERENT objects, never on the same one.
Why Do We Need It?
This law explains how things like walking, swimming, and rocket propulsion work — every one of them relies on pushing against something else in order to be pushed back in the opposite direction.
Worked Example
Identify an action-reaction pair for a swimmer
A swimmer pushes backward on the water with their arms. Identify the action-reaction pair, and explain why the swimmer moves forward.
Why Does This Work?
Forces always arise from an interaction BETWEEN two objects, so it makes sense that the interaction produces an equal and opposite effect on each of the two objects involved — never a lone force acting on just one object by itself.
Real-Life Example
Rocket propulsion
A rocket in the vacuum of space, with nothing to push against, still manages to accelerate forward.
The rocket engine pushes hot exhaust gas backward (the action); by the third law, the gas pushes the rocket forward (the reaction) — no surrounding air or ground is needed at all.
Practice
A book rests on a table. The table pushes up on the book with a normal force. What is the Newton's-third-law reaction to this force?
MediumCommon mistake
Pairing a force with another force acting on the SAME object (like weight and normal reaction on a resting book) as if they were a third-law pair — genuine action-reaction pairs always act on two different objects.
Quick Review
- For every action force, there is an equal and opposite reaction force.
- Action and reaction forces always act on two DIFFERENT objects.
- This is how walking, swimming, and rocket propulsion all work.