Work and power questions

Work and power syllabus

Questions on work and power can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 4.11 know and use the relationship between work done, force and distance moved in the direction of the force: work done = force × distance moved W = F × d
• 4.12 know that work done is equal to energy transferred
• 4.13 know and use the relationship between gravitational potential energy, mass, gravitational field strength and height: gravitational potential energy = mass × gravitational field strength × height GPE = m × g × h
• 4.14 know and use the relationship: kinetic energy = 1/2 × mass × speed² KE = 1/2 × m × v²
• 4.15 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work
• 4.16 describe power as the rate of transfer of energy or the rate of doing work
• 4.17 use the relationship between power, work done (energy transferred) and time taken: power = work done / time taken P = W / t

How to answer work and power questions

1. Name the energy stores at the start and end and state the pathway that transfers energy.
2. Separate useful output from wasted output when calculating or explaining efficiency.
3. Show the equation, substitution and unit for work, power and energy calculations.
4. When evaluating resources, compare reliability, environmental effects, cost and response time.

Work and power example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Presenting appliance power measurements

June 2022 · Paper 2P · Question 1(b)

Question 1

A student uses a watt-meter to measure the power of electrically‑operated appliances.

The student measures the mean power output (in watts) for six different appliances.

Diagram 1 shows their results.

Draw a results table for the student’s results.

Final answer

Appliance	Mean power / W
Toaster	1068
Kettle	2025
Laptop	47
Electric drill	713
Television	59
Hair dryer	1511

Mark scheme points

1. M1 Construct a results table with an appliance column and a power column, and enter the results.
2. M2 Include the correct unit in the power column heading: W.

Explanation

Put each appliance and its measured mean power in separate columns. Include watts, written as W, in the power heading so the unit applies to every value. Keep the data cells as numbers without units.

Common mistakes

• The examiner report says virtually all candidates completed a table, but a common slip was putting units in the body. Write W in the power heading, not beside each result.

Example 2: A falling suitcase and potential energy

January 2015 · Paper 2P · Question 6(b)

Question 2

A person has a suitcase with wheels.

The suitcase falls over.

Explain why it loses gravitational potential energy when it falls.

Final answer

Gravitational potential energy depends on height. As the suitcase falls, its centre of gravity becomes lower, so its height decreases and it loses gravitational potential energy.

Mark scheme points

1. M1 State that gravitational potential energy depends on height, or give GPE = mgh.
2. M2 State that the height is reduced as it falls; this can be explained by saying the centre of gravity (or centre of mass) becomes lower. Any two of the listed mark-scheme points are needed.

Explanation

For the same mass in the same gravitational field, gravitational potential energy is proportional to height, as shown by GPE = mgh. When the suitcase falls, its centre of gravity moves to a lower position, reducing its height and therefore its gravitational potential energy.

Common mistakes

• Do not just say that the suitcase “loses energy”; explain that its height, or the height of its centre of gravity, decreases.
• Do not suggest that its mass changes as it falls. The decrease in gravitational potential energy is due to the reduced height.

Practise Work and power questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.