Newtons Third Law - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Newton's Third Law

Forces

Lesson 16 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 16 of 20

Answer each one, then check.

1

What is a force?

2

What are balanced forces?

3

Give a contact force.

4

What is a non-contact force?

5

What is weight?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 16 of 20

1

What is a force?

A push or a pull

2

What are balanced forces?

Equal and opposite forces on the same object

3

Give a contact force.

Friction, tension or normal contact force

4

What is a non-contact force?

A force between objects that are not touching

5

What is weight?

The force of gravity on an object

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 16 of 20

1

State Newton's Third Law.

2

Identify pairs of forces between two interacting objects.

3

Explain why interaction pairs do not cancel each other.

4

Apply the law to objects in equilibrium.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 16 of 20

NEWTON'S THIRD LAW

Whenever two objects interact, the forces they exert on each other are equal and opposite.

The two forces in an interaction pair act on different objects, are the same type of force, and are equal in size and opposite in direction.

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Interaction Pairs

Forces

Lesson 16 of 20

Each force acts on a different object.

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Finding an Interaction Pair

Forces

Lesson 16 of 20

A book rests on a table. The table exerts an upward force on the book. What is the other force in this interaction pair?

1

Type of force

A contact (normal) force

2

Acts on

The table

3

Direction

Downwards

ANSWER

The book pushes down on the table with an equal force.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Weight and Contact Force

Forces

Lesson 16 of 20

A book at rest on a table has a weight of 10 N. Is the normal contact force from the table the Third-Law pair of the weight? Explain.

1

Weight

The Earth pulls the book down; the pair is the book pulling the Earth up

2

Normal contact force

The table pushes the book up: a different kind of force

3

So

They are equal and opposite because the book is in equilibrium, not because of the Third Law

ANSWER

No. The pair of the weight is the book's gravitational pull on the Earth. The contact force balances the weight (Newton's First Law).

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Why Things Move

Forces

Lesson 16 of 20

A swimmer pushes backwards on the water. Explain why the swimmer moves forwards.

1

Action

The swimmer pushes the water backwards

2

Reaction

The water pushes the swimmer forwards with an equal force

3

Resultant

Acts on the swimmer and causes acceleration

ANSWER

The water pushes back on the swimmer with an equal and opposite force.

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Common Mistakes

Forces

Lesson 16 of 20

Avoid these.

Same object

Forces that cancel act on the same object; a Third-Law pair acts on different objects.

Always equal

The forces in a pair are always equal, even if one object is much lighter.

Same type

The pair is always the same type of force.

Motion

Equal forces on different objects can give different accelerations (F = ma).

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Key Terms

Forces

Lesson 16 of 20

Newton's Third Law

The forces two interacting objects exert on each other are equal and opposite.

Interaction pair

The two forces in an interaction between two objects.

Equilibrium

When the forces on an object are balanced.

Normal contact force

The push from a surface on an object resting on it.

Reaction force

The force that forms a pair with another force.

Different objects

The two forces in a pair act on separate bodies.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 16 of 20

YOUR TASK

Find the Pair

10 minutes

For each, state the other force in the pair: (a) a foot pushes backwards on the ground (b) the Sun pulls the Earth (c) a rocket pushes gas out of its engine.

1

Swap the objects.

2

Keep the same type of force.

WHAT A GOOD ANSWER SHOWS

(a) The ground pushes the foot forwards. (b) The Earth pulls the Sun. (c) The gas pushes the rocket forwards.

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Can I...?

Forces

Lesson 16 of 20

State the third law.

Name the object each force acts on.

Identify a pair.

Explain that pairs do not cancel.

Apply it to a book on a table.

Apply it to a swimmer.

Apply it to gravity.

Explain equilibrium.

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Summary & Exam Focus

Interaction pairs: equal and opposite, on different objects.

Same type of force in a pair.

Weight and normal force are not a Third-Law pair.

Equilibrium: balanced forces on one object.

EXAM FOCUS

State Newton's Third Law. (2 marks)

Forces equal in size and opposite in direction on different objects.

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Exam Practice: Newton's Third Law

Forces

Lesson 16 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 16 minutes

Question 1 · 2 marks · State

State Newton's Third Law of motion.

Question 2 · 3 marks · Explain

A book rests on a table. The table pushes up on the book. State the force that forms a Newton's Third Law pair with this force, and the…

Question 3 · 3 marks · Explain

A swimmer pushes the water backwards and moves forwards. Explain why, using Newton's Third Law.

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Exam Practice: Newton's Third Law (cont.)

Forces

Lesson 16 of 20

Question 4 · 3 marks · Explain

The Earth pulls the Moon with a gravitational force. A student says that the Moon pulls on the Earth with a much smaller force because the…

Question 5 · 4 marks · Explain

A book of weight 10 N rests on a table. A student says the weight and the upward force from the table are a Newton's Third Law pair…

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Question 1 · 2 marks · State

Forces

Lesson 16 of 20

“

State Newton's Third Law of motion.

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Forces

Lesson 16 of 20

2 marks available. Award a mark for each point made.

Forces are equal

1 mark

and opposite

1 mark

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Question 2 · 3 marks · Explain

Forces

Lesson 16 of 20

“

A book rests on a table. The table pushes up on the book. State the force that forms a Newton's Third Law pair with this force, and the object it acts on.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 2 · mark scheme

Forces

Lesson 16 of 20

3 marks available. Award a mark for each point made.

Book pushes down

1 mark

On the table

1 mark

Equal in size and opposite in direction

1 mark

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Question 3 · 3 marks · Explain

Forces

Lesson 16 of 20

“

A swimmer pushes the water backwards and moves forwards. Explain why, using Newton's Third Law.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 3 · mark scheme

Forces

Lesson 16 of 20

3 marks available. Award a mark for each point made.

Swimmer pushes water backwards

1 mark

Water pushes swimmer forwards with an equal force

1 mark

Forces act on different objects or make the swimmer accelerate

1 mark

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Question 4 · 3 marks · Explain

Forces

Lesson 16 of 20

“

The Earth pulls the Moon with a gravitational force. A student says that the Moon pulls on the Earth with a much smaller force because the Moon is much less massive. Explain why the student is wrong.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 4 · mark scheme

Forces

Lesson 16 of 20

3 marks available. Award a mark for each point made.

Forces are equal

1 mark

Opposite in direction

1 mark

Newton's Third Law applies whatever the masses

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Explain

Forces

Lesson 16 of 20

“

A book of weight 10 N rests on a table. A student says the weight and the upward force from the table are a Newton's Third Law pair. Explain why the student is wrong. Name the force that does form a pair with the weight.

HOW TO ANSWER IT

Command word: Explain. Worth 4 marks, so plan before writing.

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Question 5 · mark scheme

Forces

Lesson 16 of 20

4 marks available. Award a mark for each point made.

Both forces act on the book

1 mark

Third Law pairs act on different objects

1 mark

Equal because of equilibrium

1 mark

Book pulls the Earth upwards

1 mark

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