ZIMSEC O Level · Combined Science · Form 2, Form 3
Forms of energy
Kinetic, potential (gravitational and elastic), chemical, heat, light, sound, electrical and nuclear. Energy cannot be created or destroyed, only changed from one form to another (conservation of energy).
Energy sources
- Non-renewable: coal (Hwange), oil, natural gas – they will run out and cause pollution.
- Renewable: solar, hydroelectricity (Kariba), wind, biogas and biomass.
Work and power
- Work done = force × distance moved in the direction of the force (joules, J).
- Power = work done ÷ time taken (watts, W).
- Kinetic energy = ½mv²; gravitational potential energy = mgh.
Worked example: A learner lifts a 20 kg bag of maize 1.5 m onto a cart. Find the work done (g = 10 N/kg).
Force = weight = 20 × 10 = 200 N. Work = 200 × 1.5 = 300 J
Simple machines
Levers, pulleys, inclined planes and wheel-and-axle make work easier.
- Mechanical advantage = load ÷ effort.
- Efficiency = (useful work out ÷ work in) × 100%. No machine is 100% efficient because of friction.
Exam tip: Always write the formula, substitute with units, then give the answer with its unit.
Check yourself
- Name two renewable sources of energy used in Zimbabwe.
- A machine does 600 J of work in 4 s. Find its power.
- Find the kinetic energy of a 2 kg ball moving at 3 m/s.
Show answers
- Any two: hydroelectric (Kariba), solar, biogas, wind, biomass.
- 150 W
- ½ × 2 × 9 = 9 J
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