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1.3 Energy and Work

GCSE Physics · AQA · Chapter 1: Conservation and Dissipation of Energy

A free GCSE Physics lesson for AQA (8463) from OBA School: work done by a force, W = F s, the joule, and work done against friction. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain what is meant by work done and how it is linked to energy transferred
  • Use W = F s to calculate work done, force or distance, with the correct units
  • Explain why work done against friction causes heating
  • Describe how to measure the work done dragging an object, and comment on resolution and precision

Key words

  • Work done: Energy transferred when a force moves an object. Work done = energy transferred.
  • Joule (J): The unit of energy and work. 1 J of work is done when a force of 1 N moves an object 1 m in the direction of the force.
  • Line of action: The direction in which a force acts. Only distance moved along this line counts in W = F s.
  • Friction: A force that opposes motion when two surfaces rub or slide over each other.
  • Resolution: The smallest change a measuring instrument can show, e.g. 0.1 N on a newton-meter.
  • Precision: How close repeat readings are to each other. A small spread means precise results.

What is work?

In physics, work is done when a force makes an object move. The force transfers energy to (or from) the object. The amount of energy transferred is equal to the work done:

energy transferred = work done

If you do 35 J of work lifting a bag of shopping onto a table, the gravitational potential energy store of the bag increases by 35 J. The energy came from the chemical store of your muscles.

Calculating work done

The work done depends on the size of the force and how far the object moves along the line of action of the force:

  • W = F s work done = force × distance; Rearranged F = W ÷ s s = W ÷ F; Units W in joules (J) F in newtons (N) s in metres (m)

One joule of work is done when a force of one newton moves an object one metre in the direction of the force (1 J = 1 N m). Large amounts are given in kilojoules: 1 kJ = 1000 J.

Worked example: pushing a supermarket trolley

A shopper pushes a trolley with a steady force of 35 N for 8.0 m along the aisle.
W = F s = 35 N × 8.0 m

Answer: W = 280 J

Worked example: finding a force by rearranging

A car’s kinetic energy store of 54 kJ is reduced to zero by the brakes in a distance of 18 m. Calculate the braking force.
Convert: 54 kJ = 54 000 J. The work done by the brakes = 54 000 J.
F = W ÷ s = 54 000 J ÷ 18 m

Answer: F = 3000 N

Exam tip: Distances must be in metres and energies in joules. If a force acts at right angles to the motion (for example the upward force of your hand when you carry a bag along a level corridor), that force does no work, because the bag does not move along its line of action.

Work done against friction

When work is done against friction, the energy is mainly transferred to the thermal stores of the surfaces that rub together, and then by heating to the surroundings.

  • Rubbing your hands on a cold day: your muscles do work against friction and your hands get warmer.
  • Brakes: friction between the brake pads and the spinning disc does work. Energy is transferred from the kinetic store of the vehicle to the thermal stores of the pads and disc, which then heat the air.
  • Re-entry: a space capsule falling back through the atmosphere does work against air resistance. Its kinetic and GPE stores decrease and its outer surface gets very hot, so it needs a heat shield. Small rocks from space get so hot they glow as “shooting stars”.

Investigating work done

You can measure the work done dragging a box across a bench:

  • 1. Hook a newton-meter to the box and pull it horizontally at a steady speed. Read the force while it is moving.
  • 2. Use a metre rule to measure the distance moved.
  • 3. Repeat three times and calculate a mean force. Then work out W = F s.
  • 4. Change the surface (e.g. wrap rubber bands round the box) and compare.

Worked example: processing results

Repeat readings of the force: 3.6 N, 3.8 N, 3.7 N. The box moved 0.80 m.
mean force = (3.6 + 3.8 + 3.7) ÷ 3 = 3.7 N
W = 3.7 N × 0.80 m = 2.96 J ≈ 3.0 J

Answer: about 3.0 J of work done

Practical tip: The resolution of a newton-meter marked every 0.1 N is 0.1 N. Readings that are close together are precise. Readings are accurate if they are close to the true value – check the newton-meter reads zero before you start.

Common mistakes

  • Forgetting to convert kJ to J (or cm to m) before using W = F s.
  • Using a distance that is not in the direction of the force.
  • Saying the energy from friction “disappears”. It goes to thermal stores and is dissipated.
  • Giving the unit of work as N or W. Work is measured in joules (J), the same as energy.

Revision questions

Try these, then check your answers below each one.

Question 1. A shopper pushes a trolley with a force of 40 N for 6.5 m. Calculate the work done.

Answer: 260 J

Working: W = 40 N × 6.5 m = 260 J.

Question 2. A girl does 2.4 kJ of work pulling a sledge 30 m. Calculate the force she pulls with.

Answer: 80 N

Working: 2.4 kJ = 2400 J. F = W ÷ s = 2400 ÷ 30 = 80 N.

Question 3. What is the unit of work done? (Choose from: joule, newton, watt, newton per metre.)

Answer: joule

Working: Work is measured in joules (J), the same unit as energy. 1 J = 1 N m.

Related lessons

Continue in the full course

This lesson is section 1.3 (AGCPH0103) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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