Moments levers and gears - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Moments, levers and gears

Forces

Lesson 6 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 6 of 20

Answer each one, then check.

1

What is a pivot?

2

What is the unit of force?

3

What is clockwise?

4

What is the unit of distance?

5

Give an example of a lever.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 6 of 20

1

What is a pivot?

The point about which something turns

2

What is the unit of force?

Newton

3

What is clockwise?

The direction the hands of a clock turn

4

What is the unit of distance?

Metre

5

Give an example of a lever.

A seesaw, crowbar or spanner

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 6 of 20

1

Describe examples in which forces cause rotation.

2

Recall and apply M = Fd.

3

Use the principle of moments to find an unknown force or distance.

4

Explain how levers and gears transmit the rotational effects of forces.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 6 of 20

MOMENTS

moment of a force = force × perpendicular distance from the pivot M = Fd

The turning effect of a force is called its moment, in newton-metres (Nm). If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

A Balanced Beam

Forces

Lesson 6 of 20

Clockwise moments equal anticlockwise moments when balanced.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Calculating a Moment

Forces

Lesson 6 of 20

A force of 30 N acts 1.5 m from a pivot at right angles. Calculate the moment.

1

Write the equation

M = Fd

2

Substitute

M = 30 × 1.5

3

Answer

M = 45 Nm

ANSWER

45 Nm

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A Balanced Seesaw

Forces

Lesson 6 of 20

A 40 N child sits 0.50 m from the pivot of a balanced beam. Another force F acts on the other side 0.80 m from the pivot. Calculate F.

1

Anticlockwise moment

40 × 0.50 = 20 Nm

2

Balanced

F × 0.80 = 20

3

Rearrange

F = 20 ÷ 0.80

4

Answer

F = 25 N

ANSWER

25 N

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Gears

Forces

Lesson 6 of 20

A gear with 20 teeth drives a gear with 40 teeth. Describe how the second gear turns compared with the first.

1

Direction

It turns in the opposite direction

2

Speed

The larger gear turns more slowly (half the speed)

3

Turning effect

The larger gear has a greater turning effect (moment)

ANSWER

It turns the opposite way at half the speed with a larger moment.

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Levers and Gears

Forces

Lesson 6 of 20

Transmitting turning effects.

Lever

A rigid bar that turns about a pivot; a longer distance from the pivot means a smaller force is needed.

Examples

A crowbar, spanner, wheelbarrow, scissors.

Gears

Toothed wheels that mesh to transfer rotation.

Gear ratio

A large gear driven by a small gear turns slower but with a greater moment.

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Key Terms

Forces

Lesson 6 of 20

Moment

The turning effect of a force, in newton-metres.

Pivot

The point about which an object turns.

Clockwise

In the direction of a clock's hands.

Anticlockwise

Opposite to the hands of a clock.

Lever

A bar that can turn about a pivot.

Gear

A toothed wheel that transmits rotation.

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Forces

Lesson 6 of 20

YOUR TASK

Balance the Beam

12 minutes

A beam is balanced. A 60 N weight is 0.40 m to the left of the pivot. Where must a 30 N weight be placed on the right?

1

Find the anticlockwise moment.

2

Equal the clockwise moment.

WHAT A GOOD ANSWER SHOWS

Clockwise = anticlockwise: 30 × d = 60 × 0.40 = 24, so d = 0.80 m.

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Can I...?

Forces

Lesson 6 of 20

Recall M = Fd.

Use perpendicular distance.

State the principle of moments.

Find an unknown force.

Find an unknown distance.

Explain levers.

Explain gears.

Use Nm.

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Summary & Exam Focus

M = Fd.

Balanced: total clockwise moment = total anticlockwise moment.

Levers: a longer distance from the pivot gives a greater moment.

Gears: change direction, speed and turning effect.

EXAM FOCUS

A force of 30 N acts 1.5 m from a pivot. Calculate the moment. (2 marks)

Multiply force by distance and give Nm.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Moments, levers and gears

Forces

Lesson 6 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes

Question 1 · 2 marks · Calculate

A force of 30 N is applied at a distance of 1.5 m from a pivot. The force acts at right angles. Calculate the moment of the force. Use the…

Question 2 · 3 marks · Calculate

The diagram shows a uniform beam balanced on a pivot. A 40 N force acts 0.50 m to the left of the pivot. Calculate the force F.

Question 3 · 2 marks · Explain

A spanner is used to undo a nut. Explain why a longer spanner makes it easier.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Moments, levers and gears (cont.)

Forces

Lesson 6 of 20

Question 4 · 3 marks · Explain

A gear with 20 teeth is driven by a motor. It meshes with a gear with 40 teeth. Explain how the larger gear turns compared with the smaller…

Question 5 · 4 marks · Calculate

A 60 N weight is 0.40 m to the left of a pivot on a balanced beam. A 30 N weight is on the right. Calculate its distance from the pivot.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Calculate

Forces

Lesson 6 of 20

“

A force of 30 N is applied at a distance of 1.5 m from a pivot. The force acts at right angles. Calculate the moment of the force. Use the equation: moment = force × distance

HOW TO ANSWER IT

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

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Question 1 · mark scheme

Forces

Lesson 6 of 20

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

Correct substitution

1 mark

45 Nm

1 mark

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Question 2 · 3 marks · Calculate

Forces

Lesson 6 of 20

The diagram shows a uniform beam balanced on a pivot. A 40 N force acts 0.50 m to the left of the pivot. Calculate the force F. (3 marks)

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Question 2 · mark scheme

Forces

Lesson 6 of 20

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

Anticlockwise moment 20 Nm

1 mark

Equates moments

1 mark

25 N

1 mark

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Question 3 · 2 marks · Explain

Forces

Lesson 6 of 20

“

A spanner is used to undo a nut. Explain why a longer spanner makes it easier.

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 3 · mark scheme

Forces

Lesson 6 of 20

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

Moment = force × distance

1 mark

Greater distance means smaller force needed

1 mark

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Question 4 · 3 marks · Explain

Forces

Lesson 6 of 20

“

A gear with 20 teeth is driven by a motor. It meshes with a gear with 40 teeth. Explain how the larger gear turns compared with the smaller gear.

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 4 · mark scheme

Forces

Lesson 6 of 20

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

Opposite direction

1 mark

Slower (half the speed)

1 mark

Greater moment

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Calculate

Forces

Lesson 6 of 20

“

A 60 N weight is 0.40 m to the left of a pivot on a balanced beam. A 30 N weight is on the right. Calculate its distance from the pivot.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · mark scheme

Forces

Lesson 6 of 20

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

Anticlockwise moment 24 Nm

1 mark

Balanced moments

1 mark

Correct rearrangement

1 mark

0.80 m

1 mark

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