Work Energy and Power (O-Levels)

What these notes cover

These notes cover work, energy and power for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): work done by a force and against friction, energy stores including gravitational potential and kinetic energy, the principle of conservation of energy, energy transfer pathways, power, efficiency and Sankey diagrams, and renewable and non-renewable energy resources with their advantages and disadvantages. They include worked examples and past-paper questions.

Key formulas and definitions

• Work done = force × distance moved in the direction of the force: W = Fd. Unit: joule (J). Work is a scalar.
• No work is done if there is no movement in the direction of the force; for example, the centripetal force on an object moving in a circle does no work.
• Gravitational potential energy: ΔEₚ = mgΔh, where g = 9.8 N/kg on Earth.
• Kinetic energy: Eₖ = ½mv². Doubling the speed makes the kinetic energy four times as large.
• Conservation of energy: energy cannot be created or destroyed, only transferred from one store to another. For an object falling freely without air resistance, the loss in GPE equals the gain in KE: mgh = ½mv².
• Power is the rate of doing work or transferring energy: P = W / t = E / t. Unit: watt (W), where 1 W = 1 J/s.
• Efficiency = (useful energy output ÷ total energy input) × 100% = (useful power output ÷ total power input) × 100%. A Sankey diagram shows the useful and wasted energy.
• Renewable resources (solar, wind, hydroelectric, tidal, wave, geothermal, biofuels) will not run out. Non-renewable resources (coal, oil, natural gas, nuclear fuel) are finite; burning fossil fuels releases carbon dioxide and sulfur dioxide.