Newton's Laws of Motion (AQA GCSE Physics Combined Science): Revision Notes
Newton's third law
Newton's third law is one of the most important rules about forces. It explains what happens when objects push or pull on each other, and understanding this concept is fundamental to physics and everyday life.
Newton's third law is essential for understanding how forces work in the real world. From walking and jumping to rocket propulsion and car crashes, this law explains countless phenomena we experience daily.
What is Newton's third law?
When two objects interact with each other, they create forces that are equal in size but opposite in direction. This means:
- If object A pushes object B with a force, then object B pushes back on object A with exactly the same strength
- The forces always come in pairs called action-reaction pairs
- These paired forces always act on different objects, not the same one
The simple way to remember this is: "For every action, there is an equal and opposite reaction."
Action and reaction forces
Action and reaction forces are special because they:
- Have exactly the same strength (magnitude)
- Point in opposite directions
- Act on different objects
- Happen at exactly the same time
Critical Understanding: The action and reaction forces don't cancel each other out because they act on different objects. This is a common misconception that students often have - remember, cancellation only occurs when forces act on the same object.
Common examples of Newton's third law
Let's explore some everyday examples that demonstrate how this law works in practice.
Worked Example: Person pushing a wall
When you push against a wall:
- Action force: You push the wall with your hands
- Reaction force: The wall pushes back on you with the same strength
Analysis: If you and the wall are not moving, these forces are balanced and you stay in equilibrium. The harder you push, the harder the wall pushes back!
Worked Example: Book resting on a table
When a book sits on a table:
- Action force: The book pushes down on the table due to gravity
- Reaction force: The table pushes up on the book with equal force
Why this matters: This is why the book doesn't fall through the table or float upwards. The forces are perfectly balanced.
Worked Example: Earth and objects
This law even works with gravity:
- Action force: Earth pulls down on you with gravitational force
- Reaction force: You pull up on Earth with exactly the same gravitational force
Fascinating fact: You don't notice Earth moving because it's so much more massive than you! The same force creates different accelerations due to the mass difference.
Important things to remember
Newton's third law helps explain many phenomena that might seem puzzling at first. Understanding this law reveals why:
- You move backwards when you jump off a boat - you push the boat backwards, it pushes you forwards
- Rockets can move forwards in space by pushing exhaust gases backwards, even in the vacuum of space
- You feel pushed back into your seat when a car accelerates forwards - the seat pushes you forwards, you push back on the seat
Mass Matters for Effects The forces in each pair are always equal in strength, but the effects might look different because the objects have different masses. A small force on a large mass produces little acceleration, while the same force on a small mass produces large acceleration.
Key applications in real life
This fundamental law appears everywhere in our daily lives. When you walk, your foot pushes backwards on the ground, and the ground pushes forwards on you - that's what moves you forwards. When a bird flies, its wings push air downward, and the air pushes the bird upward.
Key Points to Remember:
- Newton's third law: When objects interact, they create equal and opposite force pairs
- Action-reaction pairs: Always act on different objects at the same time
- Equal strength: The forces are always the same size but point in opposite directions
- Real examples: Pushing walls, books on tables, walking, and gravity all demonstrate this law
- Different effects: Equal forces can cause different movements if objects have different masses
- No cancellation: Action-reaction pairs don't cancel because they act on different objects