Newtons First Law - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Newton's First Law

Forces

Lesson 14 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 14 of 20

Answer each one, then check.

1

What is a resultant force?

2

What are balanced forces?

3

What is velocity?

4

What is friction?

5

What is constant velocity?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 14 of 20

1

What is a resultant force?

A single force with the same effect as all the forces

2

What are balanced forces?

Equal and opposite forces

3

What is velocity?

Speed in a given direction

4

What is friction?

A force opposing motion

5

What is constant velocity?

Same speed in the same direction

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 14 of 20

1

State Newton's First Law.

2

Apply it to objects moving at constant velocity and to objects whose speed or direction changes.

3

Explain that when a vehicle travels at a steady speed the resistive forces balance the driving force.

4

Explain inertia (Higher tier).

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 14 of 20

NEWTON'S FIRST LAW

If the resultant force on an object is zero, a stationary object remains stationary and a moving object continues at the same speed in the same direction.

The velocity of an object will only change if a resultant force acts on it.

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Balanced and Unbalanced Forces

Forces

Lesson 14 of 20

No resultant force means no change in velocity.

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Steady Speed

Forces

Lesson 14 of 20

A car travels at a steady 20 m/s. The driving force is 1500 N. What is the resistive force?

1

Steady speed

Velocity is constant, so the resultant force is zero

2

Forces balance

Resistive force = driving force

ANSWER

1500 N, in the opposite direction.

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Changing Velocity

Forces

Lesson 14 of 20

A car brakes from 20 m/s to rest. Explain the motion using Newton's First Law.

1

Velocity changes

So there must be a resultant force

2

Braking force

Greater than the driving force, acting against the motion

ANSWER

A resultant force opposes the motion, so the car decelerates.

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Inertia (Higher)

Forces

Lesson 14 of 20

Explain why a passenger moves forwards when a car brakes suddenly.

1

Before braking

The passenger moves with the car at a constant velocity

2

Braking

The car slows, but no force acts on the passenger from the seat

3

Inertia

The passenger's body tends to continue at the same velocity, so moves forward relative to the car

ANSWER

Inertia: objects tend to keep their state of rest or uniform motion.

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Key Ideas

Forces

Lesson 14 of 20

Use the law in explanations.

Zero resultant

Stationary or constant velocity.

Non-zero resultant

The velocity (speed or direction) changes.

Direction matters

A circular motion needs a resultant force towards the centre.

Inertia (Higher)

The tendency of objects to continue in their state of rest or uniform motion.

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Key Terms

Forces

Lesson 14 of 20

Newton's First Law

An object keeps its velocity unless a resultant force acts on it.

Inertia

The tendency of an object to stay at rest or keep moving at constant velocity.

Balanced forces

Forces with a resultant of zero.

Uniform velocity

Constant speed in a straight line.

Resistive force

A force opposing motion.

Driving force

A force that makes a vehicle move.

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Forces

Lesson 14 of 20

YOUR TASK

Apply the Law

10 minutes

Explain each in terms of forces: (a) a cyclist coasts at constant speed on a flat road with no pedalling (hint: think of ideal conditions) (b) a book stays on a table (c) a ball rolls to a stop on grass.

1

Is there a resultant force?

2

Does velocity change?

WHAT A GOOD ANSWER SHOWS

(a) In ideal conditions with no resistive force the resultant is zero. (b) Weight and normal contact force are balanced. (c) Friction gives a resultant force opposing the motion.

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Can I...?

Forces

Lesson 14 of 20

State the first law.

Apply it to constant velocity.

Apply it to changing velocity.

Explain balanced forces on a car.

Explain stopping in terms of forces.

Define inertia.

Explain passenger motion.

Use resultant force.

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Summary & Exam Focus

Resultant zero: stationary or constant velocity.

Resultant non-zero: velocity changes.

Steady speed: driving force equals resistive force.

Inertia: tendency to continue the same motion.

EXAM FOCUS

A car travels at a constant speed. Use Newton's First Law to explain what this tells you about the forces. (2 marks)

The resultant force is zero.

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Exam Practice: Newton's First Law

Forces

Lesson 14 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 13 minutes

Question 1 · 2 marks · State

State Newton's First Law of motion.

Question 2 · 2 marks · Explain

A car travels along a level road at a steady speed. The driving force is 1500 N. State the resistive force and explain your answer.

Question 3 · 3 marks · Explain

A lorry's driver takes his foot off the accelerator. The lorry slows down. Use Newton's First Law to explain why.

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

Forces

Lesson 14 of 20

Question 4 · 3 marks · Explain

A passenger in a car moves forwards when the car brakes suddenly. Explain why, using the idea of inertia.

Question 5 · 2 marks · Explain

A puck slides at a constant velocity across very smooth ice. Explain why it does not slow down.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · State

Forces

Lesson 14 of 20

“

State Newton's First 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 14 of 20

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

Zero resultant: stationary stays stationary

1 mark

Moving object keeps the same velocity

1 mark

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Question 2 · 2 marks · Explain

Forces

Lesson 14 of 20

“

A car travels along a level road at a steady speed. The driving force is 1500 N. State the resistive force and explain your answer.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Forces

Lesson 14 of 20

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

1500 N

1 mark

Resultant force zero

1 mark

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Question 3 · 3 marks · Explain

Forces

Lesson 14 of 20

“

A lorry's driver takes his foot off the accelerator. The lorry slows down. Use Newton's First Law to explain why.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · mark scheme

Forces

Lesson 14 of 20

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

Resistive forces remain

1 mark

Resultant force acts against the motion

1 mark

Velocity changes so the lorry slows

1 mark

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Question 4 · 3 marks · Explain

Forces

Lesson 14 of 20

“

A passenger in a car moves forwards when the car brakes suddenly. Explain why, using the idea of inertia.

HOW TO ANSWER IT

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

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Question 4 · mark scheme

Forces

Lesson 14 of 20

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

Passenger moving with the car

1 mark

Inertia: tendency to keep the same velocity

1 mark

Keeps moving forwards as the car slows

1 mark

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Question 5 · 2 marks · Explain

Forces

Lesson 14 of 20

“

A puck slides at a constant velocity across very smooth ice. Explain why it does not slow down.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · mark scheme

Forces

Lesson 14 of 20

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

Negligible friction

1 mark

Resultant force zero so velocity constant

1 mark

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