Calculate work done, gravitational potential energy, kinetic energy, and power. Ideal for GCSE physics revision with worked examples and formula triangles.
Select a calculation type below. All formulas follow GCSE physics specifications for AQA, OCR, and Edexcel.
Work is done whenever a force causes an object to move. The amount of work done equals the force applied multiplied by the distance moved in the direction of the force.
Worked Example: A person pushes a shopping trolley with a force of 30 N over a distance of 15 m. Work done = 30 × 15 = 450 J.
Worked Example: Lift a 10 kg box a height of 2 m. Force = 10 × 9.8 = 98 N. Work done = 98 × 2 = 196 J.
Any object raised above a reference point has gravitational potential energy (GPE) stored due to its position in a gravitational field.
Worked Example: A 5 kg ball is lifted 3 metres above the ground. GPE = 5 × 9.8 × 3 = 147 J. When dropped, this converts to kinetic energy (minus air resistance losses).
Any moving object has kinetic energy. The formula is:
Worked Example: A 1,200 kg car travelling at 25 m/s. KE = 0.5 × 1200 × 625 = 375,000 J (375 kJ). Note: doubling the speed quadruples the kinetic energy — this is why speed is so dangerous in road collisions.
Where k = spring constant (N/m) and e = extension (m). A spring with k = 200 N/m stretched 0.05 m stores: E = 0.5 × 200 × 0.0025 = 0.25 J.
The AQA GCSE Physics specification identifies eight energy stores. Energy is transferred between these stores via radiation, electrical work, mechanical work, or heating.
Moving objects
Hot objects
Fuel, food, batteries
Raised objects
Springs, elastic bands
Atomic nuclei
Radiation
Vibrations
The law of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another. The total energy in a closed system is always constant.
Joules are the SI unit of energy, but several other units are commonly used in different contexts.
| Unit | Symbol | Equivalent in Joules | Used for |
|---|---|---|---|
| Joule | J | 1 J | Physics calculations |
| Kilojoule | kJ | 1,000 J | Food energy, chemistry |
| Megajoule | MJ | 1,000,000 J | Industrial energy |
| Kilowatt-hour | kWh | 3,600,000 J | Electricity bills |
| Calorie (food) | kcal | 4,184 J | Nutrition labelling |
| Electron-volt | eV | 1.6 × 10⁻¹⁹ J | Atomic physics |
| British Thermal Unit | BTU | 1,055 J | Heating systems |
Try these exam-style questions. Click "Show Answer" when ready.
Work done (W) = Force (F) × Distance (d). The unit is Joules (J), where 1 Joule = 1 Newton × 1 metre. Work is only done when a force moves an object in the direction of that force. If you push against a wall and it does not move, no work is done even though you exert a force.
Gravitational potential energy (GPE) is energy stored in an object due to its height above a reference point (E = mgh). Kinetic energy (KE) is energy an object has due to its motion (E = ½mv²). When an object falls, GPE converts to KE. For a falling object with no air resistance, GPE lost = KE gained, demonstrating conservation of energy.
Energy cannot be created or destroyed — it can only be transferred from one store to another. The total energy in a closed system always remains constant. In practice, energy is often transferred to less useful forms (mainly thermal energy due to friction or air resistance), which is why real machines are never 100% efficient.
Efficiency = (useful output energy ÷ total input energy) × 100%. Typical values: LED bulbs ≈ 80–90%, electric motors ≈ 85–95%, petrol engines ≈ 25–30%, coal power stations ≈ 35%. Efficiency can also be expressed as a decimal (0 to 1) rather than a percentage. No system can be more than 100% efficient.
The work-energy theorem states that the net work done on an object equals its change in kinetic energy: Wnet = ΔKE = ½mv² − ½mu², where v is the final velocity and u is the initial velocity. This is a direct consequence of Newton's second law and is fundamental to mechanics problems at A-Level and beyond.
Work is the total energy transferred (Joules), independent of how long it takes. Power is the rate of doing work (Watts = Joules per second, P = W/t). Two engines doing the same total work but at different speeds have different power outputs. A 1,000 W (1 kW) motor doing 1,000 J of work takes exactly 1 second; a 500 W motor would take 2 seconds for the same work.