Energy conservation
Cambridge A-Level Physics (9702) · Unit 5: Work, energy and power · 8 flashcards
Energy conservation is topic 5.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 5 — Work, energy and power , alongside Gravitational potential energy and kinetic energy. In one line: Work done is the energy transferred when a force causes displacement. It's calculated as force multiplied by the displacement in the direction of the force: W = Fd cosθ (where θ is the angle between the force and displacement).
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 8 flashcards — 4 definitions, 1 key concept and 3 calculations — covering the precise wording mark schemes reward. Use the 4 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
'work done' in physics
Work done is the energy transferred when a force causes displacement. It's calculated as force multiplied by the displacement in the direction of the force: W = Fd cosθ (where θ is the angle between the force and displacement).
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand the idea of work; recall and use the equation work done = force × displacement along the force's direction.
- Recall the principle that energy is conserved and apply it.
- Recall and understand that efficiency is the useful energy output of a system divided by the total energy put in.
- Solve problems that involve efficiency.
- Define power as the rate of doing work, i.e. work done divided by the time taken.
- Use P = W / t to solve problems.
- Derive the equation P = Fv and apply it when solving problems.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Energy conservation
- › Work done against resistance = loss in GPE − gain in KE; average resistive force = that work ÷ distance travelled.
- › Use the principle of conservation of energy: Work Done against resistive forces = Loss in Gravitational Potential Energy - Gain in Kinetic Energy.
- › Define power strictly as the work done per unit time or the rate of transfer of energy.
- › Apply the principle that net power equals the output power of the engine minus the power used against friction or resistive forces.
- › Calculate thermal energy dissipation rate using P = Fv, where F is the sum of friction and air resistance.
Define 'work done' in physics.
Work done is the energy transferred when a force causes displacement. It's calculated as force multiplied by the displacement in the direction of the force: W = Fd cosθ (where θ is the angle between the force and displacement).
State the principle of conservation of energy.
The principle of conservation of energy states that energy cannot be created or destroyed; it can only be transformed from one form to another, or transferred between objects. The total energy in a closed system remains constant.
Define the efficiency of a system.
Efficiency is the ratio of useful energy output to the total energy input, often expressed as a percentage. Efficiency = (Useful energy output / Total energy input) x 100%.
A motor consumes 500J of electrical energy to lift a mass, but only 400J of potential energy is gained by the mass. What is the efficiency of the motor?
Efficiency = (Useful energy output / Total energy input) x 100% = (400J / 500J) x 100% = 80%.
Define power as work done per unit time and give its SI unit.
Power is the rate at which work is done or energy is transferred. It's calculated as P = W / t, where W is work done and t is time. The SI unit of power is the watt (W), equivalent to joules per second (J/s).
How can power be calculated if force and velocity are known?
Power can be calculated as the product of force and velocity: P = Fv, where F is the force applied and v is the velocity of the object in the direction of the force.
A car engine exerts a force of 2000 N to maintain a constant speed of 15 m/s. Calculate the power developed by the engine.
Using P = Fv, the power developed by the engine is P = 2000 N * 15 m/s = 30,000 W or 30 kW.
Describe how energy is conserved in a simple pendulum system, neglecting air resistance.
In a simple pendulum, energy continuously transforms between gravitational potential energy (GPE) at the highest point and kinetic energy (KE) at the lowest point. The total energy (GPE + KE) remains constant throughout the swing.
Review the material
Read full revision notes on Energy conservation — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 5 — Work, energy and power
Energy conservation sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Energy conservation deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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