# Newton's Third Law — GCSE Physics
Newton's Third Law is often stated as "every action has an equal and opposite reaction" — but this simple phrase hides a subtle and powerful idea. Students frequently misidentify third law pairs, making this a favourite topic for exam questions. Let's master it.
1. Newton's Third Law — The Statement
When object A exerts a force on object B, object B exerts an equal force in the opposite direction on object A.
These two forces are called an interaction pair (or action-reaction pair).
Key Properties of Interaction Pairs
The two forces in a Newton's Third Law pair always:
- Are equal in magnitude (same size)
- Act in opposite directions
- Act on two different objects
- Are the same type of force (both gravitational, both contact, etc.)
- Act simultaneously (at the same time)
2. Identifying Newton's Third Law Pairs
Example 1: Walking
When you walk:
- Your foot pushes backwards on the ground (contact force)
- The ground pushes forwards on your foot (contact force)
- These are equal in magnitude and opposite in direction
- They act on different objects (foot and ground)
This is why you can't walk on perfectly frictionless ice — the ground can't push you forward if there's no friction.
Example 2: Book on a Table
A book rests on a table:
- The book's weight pulls it down (gravitational force from Earth on book)
- The Earth is pulled up by the book (gravitational force from book on Earth) — this is the third law pair!
Common mistake: Students often say the weight of the book and the normal contact force from the table are a Newton's Third Law pair. They are NOT — because:
- They both act on the same object (the book)
- They are different types of force (gravitational vs. contact)
The actual pairs are:
- Pair 1: Weight of book (Earth pulls book down) ↔ Book pulls Earth up (gravitational)
- Pair 2: Book pushes table down (contact) ↔ Table pushes book up (normal contact)
Example 3: Swimming
A swimmer pushes water backwards with their hands → the water pushes the swimmer forwards.
Both forces are contact forces, equal in magnitude, opposite in direction, acting on different objects (swimmer and water).
Example 4: Rocket Launch
A rocket pushes exhaust gases downwards → the exhaust gases push the rocket upwards.
This is how rockets work in the vacuum of space — they don't need air to push against. The interaction is between the rocket and its exhaust.
3. Why Don't the Forces Cancel Out?
A common question: if the forces are equal and opposite, why don't they cancel?
Answer: The forces act on different objects. Forces can only cancel (give zero resultant) if they act on the same object.
- When you push a wall: you push the wall (force on wall), the wall pushes you (force on you). These forces don't cancel because they act on different things.
- If you push a box and friction pushes back — both forces are on the same box, so they can result in zero net force. But this is NOT Newton's Third Law; it's just balanced forces.
4. Newton's Third Law vs. Balanced Forces
| Property | Newton's Third Law Pair | Balanced Forces |
|---|---|---|
| Number of objects | Two different objects | One object |
| Same type of force? | Always yes | Can be different types |
| Always equal? | Always yes | Only when acceleration = 0 |
| Direction | Always opposite | Always opposite |
| When do they exist? | Always — as long as the force exists | Only when resultant = 0 |
Example to clarify:
A person standing on the ground:
- Balanced forces on the person: Weight (down) = Normal force (up) → resultant = 0, person doesn't accelerate
- Newton's Third Law pairs:
- Earth pulls person down ↔ Person pulls Earth up (gravitational pair)
- Person pushes ground down ↔ Ground pushes person up (contact pair)
Worked Example: Example 1
Question: A hammer hits a nail with a force of 500 N. Describe the Newton's Third Law pair.
The hammer exerts a force of 500 N on the nail (downward/forward). By Newton's Third Law, the nail exerts a force of 500 N on the hammer in the opposite direction (upward/backward). Both are contact forces.
Worked Example: Example 2
Question: Two ice skaters, A (mass 50 kg) and B (mass 80 kg), face each other. A pushes B with a force of 100 N. (a) What force does B exert on A? (b) Calculate the acceleration of each skater. (c) Why do they accelerate at different rates?
(a) By Newton's Third Law, B exerts a force of 100 N on A in the opposite direction.
(b) Acceleration of A: m/s² Acceleration of B: m/s²
(c) Although the forces are equal (Third Law), the accelerations are different because the skaters have different masses. From , the lighter skater (A) experiences a greater acceleration.
Worked Example: Example 3
Question: Explain how Newton's Third Law allows a jet engine to propel an aircraft forward.
The jet engine pushes exhaust gases backwards with a large force. By Newton's Third Law, the exhaust gases push the engine (and aircraft) forwards with an equal force. This forward force is called thrust and propels the aircraft.
6. Practice Questions
- State Newton's Third Law of Motion. (2 marks)
- Identify the Newton's Third Law pair for each: (a) A ball falling through air (consider the gravitational force) (2 marks) (b) A foot kicking a football (2 marks)
- Explain why a book resting on a table involves two Newton's Third Law pairs, not one. (4 marks)
- Two objects collide. Object X (mass 3 kg) exerts a force of 150 N on object Y. (a) State the force object Y exerts on object X. (1 mark) (b) If object Y has a mass of 5 kg, calculate the acceleration of each object during the collision. (4 marks)
- A student says: "The weight of a book and the normal contact force from the table are Newton's Third Law pairs because they are equal and opposite." Explain why this is incorrect. (3 marks)
Answers
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Frequently Asked Questions
If a truck hits a small car, doesn't the truck exert more force?
No! Newton's Third Law guarantees the forces are equal. The truck exerts the same force on the car as the car exerts on the truck. The car suffers more damage because it has less mass and therefore experiences greater acceleration (and deceleration) — not because the force is bigger.
Does Newton's Third Law apply to non-contact forces?
Yes! Gravitational, electrostatic, and magnetic forces all obey Newton's Third Law. The Earth pulls you down; you pull the Earth up with the same force. A magnet attracts a fridge; the fridge attracts the magnet.
Can I use Newton's Third Law to explain why things don't move?
Be careful — objects being stationary is usually explained by balanced forces (Newton's First Law), not Newton's Third Law. The Third Law explains the pairs of forces between two objects.
Summary
- Newton's Third Law: Every force has an equal and opposite reaction force
- Interaction pairs act on two different objects and are the same type of force
- The forces are always equal in magnitude and opposite in direction
- They do not cancel because they act on different objects
- Don't confuse Third Law pairs with balanced forces on a single object
- Applications: walking, swimming, rockets, collisions