2.2.6 Newton's Third Law — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

Section 1 of 9The law

Newton's third law says that when object A exerts a force on object B, object B exerts an equal and opposite force on object A. The two forces are the same type, have the same size, act along the same line, point in opposite directions, and act on different objects. They are called an interaction pair or action–reaction pair.

The third law holds for every kind of force, for contact forces such as push and pull and for non-contact forces such as gravity, magnetism and electrostatics. It holds whether the objects are moving or at rest, and whether they are large or small. The two forces appear and disappear at the same instant.

Section 2 of 9Identifying pairs

To find the pair of a force, swap the two objects in the description:

1

The Earth pulls the book down (gravity) — the book pulls the Earth up with an equal force.

2

A foot pushes backwards on the ground — the ground pushes forwards on the foot (friction), which moves the walker forwards.

3

A rocket pushes gas backwards — the gas pushes the rocket forwards.

4

A swimmer pushes water backwards — water pushes the swimmer forwards.

A neat test is to ask what the pair would do if the objects were swapped.

1

If you push a wall with your hand, the wall pushes your hand.

2

Both forces are contact (electromagnetic) forces of the same size.

3

You feel the wall's force on you, which is why your hand can hurt if you push too hard.

4

Another everyday case is a person standing on a boat who steps onto the shore: the boat moves backwards because the person pushes it backwards.

Section 3 of 9Equal and opposite is not the same as balanced

The two forces of a third-law pair act on different objects, so they can never cancel each other.

Equal and opposite is not the same as balanced

Balanced forces, on the other hand, act on the same object.

A book on a table has weight (Earth on book) and a normal contact force (table on book): they are equal and opposite and balance,

but they are not a third-law pair,
because they are different kinds of force and act on the same object.

Another way to avoid confusion is to write each force as "force by X on Y". The weight is "force by Earth on book", its pair is "force by book on Earth". If your two forces share the same "on" object, they are not a third-law pair.

Section 4 of 9Why things still accelerate

If a horse pulls a cart and the cart pulls back equally on the horse, why does the cart move?

1

The two forces act on different objects.

2

For the cart, the pull forwards from the horse is bigger than the friction backwards, so it accelerates.

3

For the horse, the push from the ground forwards is greater than the cart's backward pull.

4

Always draw only the forces on one object when working out its motion.

Section 5 of 9Collisions

When two objects collide, each exerts the same size of force on the other, for the same time.

Collisions

Each therefore receives the same impulse and the changes in momentum are equal and opposite.

This is how conservation of momentum arises from the third law.

A small car and a lorry exert equal forces on each other in a crash, but the car suffers a bigger acceleration because its mass is smaller (a = F/m).

Section 6 of 9Examples to remember

Recoil of a gun, the push of a jet engine, walking and the swimmer's push all rely on the third law. A balloon released with the neck open flies forwards as air rushes backwards. Compare with Newton's first and second laws from the earlier lessons for a complete picture of motion and force.

Walking shows the law clearly. Your foot pushes backwards on the ground. The ground pushes forwards on your foot with a friction force. On smooth ice the foot slides backwards and the push is small, so walking is difficult. Rowing and propellers likewise rely on pushing a medium backwards.

Section 7 of 9Worked examples

A 70 kg person stands on the floor. Name the force pair involving the person's weight.
A 1000 kg car and a 5000 kg lorry collide head-on. The car feels a force of 40 kN. State the force on the lorry and compare the accelerations.
Explain how a swimmer moves forward.

Section 8 of 9Key terms

Term	Meaning
Newton's third law	Forces between two objects are equal and opposite.
Interaction pair	The two forces of a third-law pair.
Reaction	The partner force acting on the other object.
Recoil	Backward motion due to the reaction force.
Thrust	Forward force from expelled material, such as in a rocket.

Section 9 of 9Exam technique

1

Thinking that the forces in a pair cancel out.

2

Saying the heavier object exerts a larger force in a collision.

3

Choosing weight and normal reaction as a third-law pair.

4

Believing an interaction pair can be of different types, such as gravity and contact.

5

Forgetting which object each force acts on.