Edexcel GCSE Physics Topic 8 Energy - Forces doing work: a complete overview of work done, power, energy stores and system changes, and efficiency

What Topic 8 actually demands

Energy - Forces doing work is a Paper 2 topic that applies the energy ideas of Topic 3 to forces, with a strong calculation focus on work done, power and efficiency. It rewards fluent multi-step calculation (especially work done then power) and clear explanation of dissipation and efficiency.

This guide walks through all four dot points of the topic, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Work done and energy transfer

Work done is the energy transferred when a force moves an object, $E = F \times d$ (distance in the direction of the force), in joules. The energy transferred equals the work done, and one joule is one newton metre. Work done against friction transfers energy to the thermal store, so the surfaces warm up.

Power

Power is the rate of energy transfer (or rate of doing work), $P = \frac{E}{t}$, in watts, where one watt is one joule per second. A more powerful device transfers the same energy in less time. The personal-power core practical measures the work done climbing stairs (weight times vertical height) divided by the time.

Energy stores and system changes

A system's energy changes by work done, electrical work, heating or radiation. In a closed system the total energy is constant: energy moves between stores but is never created or destroyed. When forces do work, some energy is always dissipated to the thermal store of the surroundings, raising the temperature.

Efficiency of forces

The efficiency of a machine is the useful output over the total input, with no unit, multiplied by $100$ for a percentage. A machine is never $100\%$ efficient because friction wastes energy as heat. Lubrication (and streamlining) reduces this waste, raising the efficiency.

How Topic 8 is examined

A typical Edexcel profile for Energy - Forces doing work:

• Calculations. Work done, power (including two-stage work-then-power problems) and efficiency.
• Core practical. Measuring personal power from stair climbing.
• Explanations. Why machines waste energy as heat, and how to reduce it.
• Energy stores. Describing transfers and dissipation when forces act.

Check your knowledge

A mix of recall and calculation questions covering Topic 8. Attempt them under timed conditions, then check against the solutions.

1. State the work done equation. (1 mark)
2. A force of $30\,\text{N}$ moves an object $4\,\text{m}$ in the direction of the force. Calculate the work done. (2 marks)
3. State what one watt is equal to. (1 mark)
4. A motor transfers $2400\,\text{J}$ in $12\,\text{s}$. Calculate its power. (2 marks)
5. A student of weight $500\,\text{N}$ climbs stairs of height $3\,\text{m}$ in $4\,\text{s}$. Calculate the power. (3 marks)
6. Name two ways the energy of a system can be changed. (2 marks)
7. A machine does $80\,\text{J}$ of useful work from $200\,\text{J}$ supplied. Calculate the efficiency. (2 marks)
8. State why a machine with moving parts is never 100% efficient. (1 mark)