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Physics · Forces

Work done and energy transfer

Use \(W = Fs\) to calculate work done, describe the energy transfers, and explain why work done against friction raises temperature.

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is energy measured in?

    Show answerHide answer

    Joules

  2. 2

    What is a force measured in?

    Show answerHide answer

    Newtons

  3. 3

    What is distance measured in?

    Show answerHide answer

    Metres

  4. 4

    What is friction?

    Show answerHide answer

    A force opposing motion

  5. 5

    Name an energy store that increases when something warms up.

    Show answerHide answer

    Thermal

Learning Objectives

  1. 1Define work done.
  2. 2Recall and apply \(W = Fs\).
  3. 3Convert between newton-metres and joules.
  4. 4Describe the energy transfers when work is done, including against friction.

WORK DONE

work done \(=\) force \(\times\) 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.

Calculating Work Done

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

Show the solutionHide the solution
  1. 1 Write the equation \(W = Fs\)
  2. 2 Substitute \(W = 50 \times 12\)
  3. 3 Answer \(W = 600\) J

Answer600 J

Finding the Force

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

Show the solutionHide the solution
  1. 1 Rearrange \(F = W \div s\)
  2. 2 Substitute \(F = 1200 \div 8.0\)
  3. 3 Answer \(F = 150\) N

Answer150 N

Work Against Friction

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

Show the solutionHide the solution
  1. 1 Friction force The brake pads rub on the wheel
  2. 2 Work done Work is done against the frictional force
  3. 3 Energy transfer Energy is transferred from the kinetic store to the thermal store of the brakes

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

Key Ideas

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.

Work It Out

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.

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.

Can I...?

  1. 1Define work done.
  2. 2Use \(W = Fs\).
  3. 3Rearrange for F or s.
  4. 4Convert Nm to J.
  5. 5Describe energy transfer.
  6. 6Explain heating due to friction.
  7. 7Use the unit joule.
  8. 8Show the working.

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) (2 marks)

Multiply force by distance and give J.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

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.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Calculate 2 marks Easier

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

Mark scheme — 2 marks available

  • Correct substitution — 1 mark
  • 600 J — 1 mark

Model answer

\(50 \times 12 = 600\) J

2. Exam question Calculate 3 marks Easier

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

Mark scheme — 3 marks available

  • Rearranges to F = W ÷ s — 1 mark
  • Correct substitution — 1 mark
  • 150 N — 1 mark

Model answer

\(F = W \div s = 1200 \div 8.0 = 150\) N

3. Exam question Explain 2 marks Easier

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

Mark scheme — 2 marks available

  • Work done against friction — 1 mark
  • Kinetic to thermal energy store — 1 mark

Model answer

Work is done against the friction between the pads and the wheel. The kinetic energy of the bicycle is transferred to the thermal energy store of the brakes.

4. Exam question Describe 2 marks Easier

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

Mark scheme — 2 marks available

  • Force of 1 N — 1 mark
  • Moves an object 1 m — 1 mark

Model answer

One joule is the work done when a force of one newton moves an object a distance of one metre in the direction of the force.

5. Exam question Calculate 4 marks Easier

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.

Mark scheme — 4 marks available

  • Force = weight = mg — 1 mark
  • 1960 N — 1 mark
  • Correct substitution into W = Fs — 1 mark
  • 29 400 J — 1 mark

Model answer

\(F = 200 \times 9.8 = 1960\) N; \(W = 1960 \times 15 = 29\,400\) J

6. Multiple choice 1 mark Easier

Work done is measured in...

  1. A joules Correct
  2. B newtons
  3. C watts
  4. D metres

Why: The same unit as energy.

7. Multiple choice 1 mark Core

A force of 10 N moves an object 3 m. Work done is...

  1. A 13 J
  2. B 30 J Correct
  3. C 3.3 J
  4. D 0.3 J

Why: 10 × 3.

8. Multiple choice 1 mark Core

1 joule equals...

  1. A 1 newton per metre
  2. B 1 watt
  3. C 1 newton-metre Correct
  4. D 1 second

Why: Nm.

9. Multiple choice 1 mark Core

Work done against friction causes...

  1. A a fall in temperature
  2. B no change
  3. C an increase in mass
  4. D a rise in temperature Correct

Why: Energy transferred to the thermal store.

10. Multiple choice 1 mark Stretch

To find the force you use...

  1. A \(F = W \div s\) Correct
  2. B \(F = W \times s\)
  3. C \(F = s \div W\)
  4. D \(F = W + s\)

Why: Rearrange the equation.