ZIMSEC O Level · Physics · Form 3
Energy
Energy is measured in joules (J). It cannot be created or destroyed, only transferred (conservation of energy).
- Kinetic energy: KE = ½mv².
- Gravitational potential energy: GPE = mgh.
Work and power
- Work done = force × distance moved in the direction of the force (W = Fd).
- Power = work done ÷ time = energy transferred ÷ time (W).
- Efficiency = useful energy output ÷ total energy input × 100%.
Worked example: A 2 kg stone falls from 20 m. Find its speed just before hitting the ground (ignore air resistance).
GPE lost = mgh = 2 × 10 × 20 = 400 J = KE gained
½ × 2 × v² = 400 → v² = 400 → v = 20 m/s
Energy resources
Non-renewable: coal (Hwange), oil, gas, nuclear. Renewable: hydroelectric (Kariba), solar, wind, biomass, biogas. Consider cost, reliability and pollution.
Exam tip: In energy calculations, mass must be in kg and speed in m/s.
Check yourself
- A pump lifts 500 N of water through 6 m in 10 s. Find its power.
- Find the kinetic energy of a 1 000 kg car moving at 10 m/s.
- A motor uses 2 000 J and does 1 500 J of useful work. Find its efficiency.
Show answers
- 3 000 J ÷ 10 s = 300 W
- 50 000 J
- 75%
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