Work done and energy transfer - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Work done and energy transfer

Forces

Lesson 4 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 4 of 20

Answer each one, then check.

1

What is energy measured in?

2

What is a force measured in?

3

What is distance measured in?

4

What is friction?

5

Name an energy store that increases when something warms up.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 4 of 20

1

What is energy measured in?

Joules

2

What is a force measured in?

Newtons

3

What is distance measured in?

Metres

4

What is friction?

A force opposing motion

5

Name an energy store that increases when something warms up.

Thermal

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 4 of 20

1

Define work done.

2

Recall and apply W = Fs.

3

Convert between newton-metres and joules.

4

Describe the energy transfers when work is done, including against friction.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 4 of 20

WORK DONE

work done = force × distance moved along the line of action of the force W = Fs

One joule of work is done when a force of one newton moves an object one metre. 1 J = 1 Nm.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Work Done

Forces

Lesson 4 of 20

Work done is energy transferred by a force.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Calculating Work Done

Forces

Lesson 4 of 20

A force of 50 N pushes a box 12 m along the floor. Calculate the work done.

1

Write the equation

W = Fs

2

Substitute

W = 50 × 12

3

Answer

W = 600 J

ANSWER

600 J

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Finding the Force

Forces

Lesson 4 of 20

A crate is moved 8.0 m by a force. The work done is 1200 J. Find the force.

1

Rearrange

F = W ÷ s

2

Substitute

F = 1200 ÷ 8.0

3

Answer

F = 150 N

ANSWER

150 N

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Work Against Friction

Forces

Lesson 4 of 20

Explain why the brakes on a bicycle get warm when it slows down.

1

Friction force

The brake pads rub on the wheel

2

Work done

Work is done against the frictional force

3

Energy transfer

Energy is transferred from the kinetic store to the thermal store of the brakes

ANSWER

The kinetic energy of the bicycle is transferred to the thermal energy of the brakes and wheel.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Ideas

Forces

Lesson 4 of 20

Learn these links.

Work is energy transferred

When a force moves an object, energy is transferred.

Distance

Measured in the direction of the force.

Friction

Work done against friction raises the temperature of the object.

Units

Convert kJ to J before calculating.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Terms

Forces

Lesson 4 of 20

Work done

Energy transferred when a force moves an object.

Joule

The unit of work and energy.

Newton-metre

A joule: one newton moving an object one metre.

Friction

A force between surfaces that opposes motion.

Displacement

The distance moved in a particular direction.

Dissipated

Transferred to less useful stores.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 4 of 20

YOUR TASK

Work It Out

10 minutes

A student pushes a trolley with a force of 40 N for 6.0 m. How much work is done? What happens to the energy if the trolley is moving at a steady speed on a rough floor?

1

Calculate the work.

2

Describe the energy transfer.

WHAT A GOOD ANSWER SHOWS

W = 40 × 6.0 = 240 J. The energy is transferred to the thermal store of the trolley and floor by friction.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Can I...?

Forces

Lesson 4 of 20

Define work done.

Use W = Fs.

Rearrange for F or s.

Convert Nm to J.

Describe energy transfer.

Explain heating due to friction.

Use the unit joule.

Show the working.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

W = Fs.

1 J = 1 Nm.

Work done against friction raises the temperature.

Work is energy transferred.

EXAM FOCUS

A force of 50 N moves a box 12 m. Calculate the work done. (2 marks)

Multiply force by distance and give J.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Work done and energy transfer

Forces

Lesson 4 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 14 minutes

Question 1 · 2 marks · Calculate

A person pushes a box with a force of 50 N. The box moves 12 m. Calculate the work done. Use the equation: work done = force × distance

Question 2 · 3 marks · Calculate

A force does 1200 J of work moving a crate 8.0 m. Calculate the force.

Question 3 · 2 marks · Explain

A bicycle rider brakes and the brake pads become warm. Explain why.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Work done and energy transfer (cont.)

Forces

Lesson 4 of 20

Question 4 · 2 marks · Describe

State what one joule of work means in terms of force and distance.

Question 5 · 4 marks · Calculate

A crane lifts a 200 kg load through 15 m at constant speed. Gravitational field strength = 9.8 N/kg. Calculate the work done on the load by…

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Calculate

Forces

Lesson 4 of 20

“

A person pushes a box with a force of 50 N. The box moves 12 m. Calculate the work done. Use the equation: work done = force × distance

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · mark scheme

Forces

Lesson 4 of 20

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

Correct substitution

1 mark

600 J

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 3 marks · Calculate

Forces

Lesson 4 of 20

“

A force does 1200 J of work moving a crate 8.0 m. Calculate the force.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Forces

Lesson 4 of 20

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

Rearranges to F = W ÷ s

1 mark

Correct substitution

1 mark

150 N

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 2 marks · Explain

Forces

Lesson 4 of 20

“

A bicycle rider brakes and the brake pads become warm. Explain why.

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 4 of 20

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

Work done against friction

1 mark

Kinetic to thermal energy store

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 2 marks · Describe

Forces

Lesson 4 of 20

“

State what one joule of work means in terms of force and distance.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · mark scheme

Forces

Lesson 4 of 20

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

Force of 1 N

1 mark

Moves an object 1 m

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Calculate

Forces

Lesson 4 of 20

“

A crane lifts a 200 kg load through 15 m at constant speed. Gravitational field strength = 9.8 N/kg. Calculate the work done on the load by the crane.

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 4 of 20

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

Force = weight = mg

1 mark

1960 N

1 mark

Correct substitution into W = Fs

1 mark

29 400 J

1 mark

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