Lesson 5 of 7 · Mechanics and materials
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Energy is the most useful tool in A-Level mechanics. An energy argument is often the quickest route through a slope, lift, vehicle or spring problem, and it comes back in the synoptic final parts of fields, circuits and thermal questions. Every board tests it the same three ways: quick multiple-choice calculations (units, , areas under graphs), multi-step “calculate” and “show that” items about vehicles, cranes and slopes, and practical questions on measuring efficiency.
3 questions on work, energy, power and efficiency. Every option is explained once you answer.
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Work done by a force at angle θ to the displacement?
Work is done when a force moves its point of application. Only the component of the force along the displacement does work:
Here is the angle between the force and the displacement. Work is a scalar measured in joules: .
| Store | Equation | Notes |
|---|---|---|
| Kinetic | Doubling gives four times the | |
| Gravitational potential | Only in a uniform field, near a planet’s surface; is the vertical height change | |
| Elastic potential |
Energy cannot be created or destroyed. For any system, use this working equation:
Derivations (learn these by heart):
Power is the rate of doing work or transferring energy: , measured in watts (). For a force moving an object at speed :
Multiply by 100% for a percentage. Efficiency can never be greater than 1. The rest of the energy is dissipated, usually as thermal energy in the surroundings (through friction, air resistance or the heating effect of a current) and sometimes as sound.
| Only up to the limit of proportionality |