Newton's third law (AQA GCSE Physics): Revision Notes
Newton's third law
What is Newton's third law?
When two objects interact with each other, they both create forces on each other. These forces are always equal in size but point in opposite directions.
This means that for every action, there is always an equal and opposite reaction.
Newton's third law is one of the fundamental principles of physics that explains how forces work in our everyday world. Understanding this law helps explain everything from walking to rocket propulsion.
Action and reaction forces
Newton's third law tells us that forces always come in pairs called action-reaction pairs. Here's what you need to know:
- The action force and reaction force are always the same size
- They always point in opposite directions
- They act on different objects (this is very important!)
Critical Concept: Action and reaction forces never act on the same object! This is the most common misconception students have about Newton's third law. The forces always involve two separate objects interacting with each other.
Example: Person pushing a wall
Worked Example: Person Pushing Against a Wall
When someone pushes against a wall:
- Action force: The person pushes the wall with 100N force
- Reaction force: The wall pushes back on the person with 100N force
- Result: Both person and wall stay still because the forces are balanced
Note that the action force acts on the wall, while the reaction force acts on the person - they're acting on different objects!
Common examples of Newton's third law
Understanding Newton's third law becomes easier when we look at everyday examples. Each situation demonstrates how action-reaction pairs work in the real world.
Walking
Worked Example: How Walking Works
- Action: Your foot pushes backwards on the ground
- Reaction: The ground pushes forwards on your foot
- Result: This reaction force from the ground propels you forwards
Without friction between your foot and the ground, walking would be impossible!
Book on a table
When a book sits on a table, two forces create an action-reaction pair:
- The book pushes down on the table due to gravity (action)
- The table pushes up on the book with equal force (reaction)
- The book stays still because these forces balance
Gravitational pull
This example shows Newton's third law working on a massive scale:
- Earth pulls down on objects with gravitational force (action)
- Objects pull up on Earth with equal force (reaction)
- We don't notice Earth moving because it's so incredibly massive compared to everyday objects
Important points to remember
Forces act on different objects: Action and reaction forces never act on the same object. They always involve two different things interacting with each other. This is what makes Newton's third law different from simply having balanced forces.
Not the same as balanced forces: Don't confuse Newton's third law with balanced forces. Balanced forces act on the same object and cancel each other out, while action-reaction pairs act on different objects.
Remember!
Key Points to Remember:
- For every action, there is an equal and opposite reaction
- Action-reaction pairs are always equal in size but opposite in direction
- These force pairs act on different objects, not the same one
- Newton's third law explains how we walk, drive cars, and why rockets work
- The forces don't cancel each other out because they act on different things