Newton's Third Law

Newton's Third Law Statement

Newton's Third Law states that for every action force, there is an equal and opposite reaction force. This means:

• The action and reaction forces have the same magnitude (size).
• They act in opposite directions.
• They act on different objects, never on the same object.

For example, if object A exerts a force on object B, then object B simultaneously exerts a force of equal size but in the opposite direction on object A.

It is important to remember that these forces always come in pairs, called action-reaction pairs. Neither force exists without the other.

Examples of Newton's Third Law

Newton's Third Law can be seen in many everyday situations and physical phenomena:

Forces in Collisions

When two objects collide, such as two cars crashing or a football hitting a wall, the force that object A exerts on object B is matched by an equal and opposite force from object B on object A. This explains why both objects experience forces during the collision.

Rocket Propulsion

A rocket moves forward by pushing exhaust gases backwards. The rocket exerts a force on the gases, and the gases exert an equal and opposite force on the rocket, propelling it forward. This is how rockets work in space where there is no air to push against.

Walking Forces

When you walk, your foot pushes backwards against the ground. According to Newton's Third Law, the ground pushes your foot forwards with an equal and opposite force. This forward force from the ground moves you ahead.

Swimming Forces

When swimming, your hands push water backwards. The water pushes your hands forwards with an equal and opposite force, allowing you to move through the water.

For instance, when a swimmer pushes water backwards with a force of 50 N, the water pushes the swimmer forwards with a force of 50 N.

Implications of Newton's Third Law

Newton's Third Law has several important implications:

• Forces always come in pairs: You cannot have a force acting on one object without a corresponding force acting on another object.
• No single isolated force: Every force has a partner force acting in the opposite direction on a different object.
• Explains interactions between objects: It helps us understand how objects push or pull on each other and how motion results from these interactions.

This law is fundamental in analysing forces in systems, whether stationary or moving.

For example, when a book rests on a table, the book exerts a downward force on the table due to its weight. The table exerts an equal and opposite upward force on the book, supporting it. These two forces form an action-reaction pair.

Example: Forces in a Collision

Two ice skaters push off each other. Skater A exerts a force of 80 N on Skater B. What force does Skater B exert on Skater A?

According to Newton's Third Law, Skater B exerts an equal and opposite force on Skater A:

$F_B = -F_A = -80 \text{ N}$

The negative sign shows the force is in the opposite direction.