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Newton's Third Law states that whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. Free notes.
Newton's Third Law: Action and Reaction is part of Forces and motion in GCSE AQA Higher Physics.
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Newton's Third Law states that whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. Free notes.
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Use this lesson to stay inside Forces and motion while connecting the detail back to the wider Forces topic.
Read the core explanations from Newton's Third Law: Action and Reaction before testing yourself from memory.
Picture yourself walking across the room right now. You push backwards on the floor with your foot, and somehow you move forwards. How does this work?
The answer lies in one of physics' most elegant principles: Newton's Third Law. Every time you interact with something, it interacts back with exactly the same strength.
This isn't just about walking. When you sit on a chair, lean against a wall, or even breathe, Newton's Third Law is working behind the scenes. Understanding this law unlocks how rockets launch, why you don't fall through the floor, and how birds fly.
Notice the key word: 'interact'. Forces don't exist in isolation. They always come in pairs, acting on different objects.
Think of forces like dance partners. When one dancer pushes, the other feels an equal push back. In physics, we call these action-reaction pairs.
Here's the crucial point: action-reaction forces act on different objects. When you push on a wall, the wall pushes back on you with equal force. You feel the wall's force, but the wall feels your force.
**Trap**: Many students think action-reaction forces cancel out because they're equal and opposite. They don't cancel because they act on different objects. Forces only cancel when they act on the same object.
Consider a book resting on a table. The book pushes down on the table with its weight. The table pushes up on the book with an equal force. These forces don't cancel each other out - they act on different objects.
Newton's Third Law becomes particularly important when objects are in equilibrium. This is when all forces acting on an object are perfectly balanced.
Consider a person standing still on the ground. Their weight pulls them down with force $W$. The ground pushes up with an equal force $N$ (called the normal force). These forces act on the same object (the person), so they cancel out, creating equilibrium.
But there's also a Newton's Third Law pair here. The person pushes down on the ground with force $W$, and the ground pushes up on the person with force $N$. These forces are equal in magnitude but act on different objects.
This is where students often get confused. There are actually four forces in this situation: two acting on the person (weight down, normal up) and two acting on the ground (person's push down, ground's reaction up).
Newton's Third Law makes force calculations straightforward. If you know the action force, you automatically know the reaction force - it's equal in magnitude and opposite in direction.
Worked example: Finding Reaction Forces
A 2kg book sits on a table. The table is on the ground. Calculate all the forces in this system. (Take g = 10 N/kg)
Answer: Six forces total: Book's weight (20 N down), table's force on book (20 N up), book's force on table (20 N down), ground's force on table (20 N up), table's force on ground (20 N down), and ground's reaction (20 N up into Earth).
Notice how Newton's Third Law doubles the number of forces we consider. For every force we identify, there's always an equal and opposite partner acting on the other object.
Newton's Third Law explains countless everyday phenomena. When you walk, you push backwards on the ground, and the ground pushes you forwards. Without friction, this wouldn't work - try walking on ice!
Birds fly by pushing air downwards with their wings. The air pushes back upwards, creating lift. Fish swim by pushing water backwards with their fins and tail, while the water pushes them forwards.
Rocket propulsion is perhaps the most dramatic example. Rockets work by expelling hot gas at high speed in one direction. The gas pushes back on the rocket in the opposite direction. This works perfectly in the vacuum of space because rockets don't need air to 'push against' - they push against their own exhaust.
Even when you're sitting still, Newton's Third Law is at work. You push down on your chair, and your chair pushes up on you. Your chair pushes down on the floor, and the floor pushes up on your chair. It's action-reaction pairs all the way down!
Exam questions on Newton's Third Law often test whether you can identify action-reaction pairs correctly. The key is remembering that these forces always act on different objects.
When describing action-reaction pairs, examiners want you to be specific about which object exerts the force and which object experiences it. Don't just say 'equal and opposite forces' - identify what's pushing what.
For equilibrium questions, draw clear force diagrams showing all forces acting on each object separately. This helps you avoid the common mistake of mixing up forces acting on different objects.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Newton's Third Law states that whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. These forces always come in pairs and act on different objects.
Action and reaction are the two forces in Newton's Third Law: whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. They always come in pairs and act on different objects.
An action-reaction pair is two forces that are equal in magnitude, opposite in direction and act on different objects during an interaction. Pushing on a wall makes the wall push back with an equal force.
To identify an action-reaction pair, state which object exerts each force and which object experiences it, such as: the book pushes on the table, and the table pushes on the book. Drawing separate force diagrams for each object helps avoid mixing up forces.
Rockets expel hot gas at high speed in one direction, and the gas pushes back on the rocket in the opposite direction. This works in the vacuum of space because rockets do not need air to push against; they push against their own exhaust.
A book resting on a table has its weight acting downwards and an equal upward force from the table, so it is in equilibrium. For a 2kg book with g = 10 N/kg, the weight is 20 N, the table pushes up with 20 N, and the book pushes down on the table with 20 N.
A person standing still has two forces on them: weight down and the normal force from the ground up, which cancel. There is also a Newton's Third Law pair: the person pushes down on the ground and the ground pushes up on the person, so there are four forces in total.
Birds fly by pushing air downwards with their wings. By Newton's Third Law, the air pushes back upwards on the wings with an equal force, creating lift.
Rockets work in the vacuum of space because they do not need air to push against; they push against their own exhaust. Expelling hot gas at high speed in one direction makes the gas push back on the rocket in the opposite direction.
A 2 kg book on a table, with g = 10 N/kg, has a weight of 20 N down and a 20 N upward force from the table. By Newton's Third Law, the book pushes down on the table with 20 N, and the table and the ground push on each other with 20 N too.
People do not fall through the floor because the floor pushes up on them with a force equal to their weight. A person pushes down on the floor, and the floor pushes up on the person with an equal and opposite force.
Balanced forces act on the same object and cancel out, like weight and the normal force on a standing person. A Newton's Third Law pair acts on different objects, so the forces do not cancel even though they are equal and opposite.