IGCSE Physics revision · Mechanics
Energy, work and power
The topic that connects everything: a falling object is a motion question, an energy question and often an efficiency question in one. Use the energy-store and transfer terms required by your syllabus, and explain the physical change.
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What the syllabus demands
- —Describe energy stores (kinetic, gravitational potential, chemical, elastic, internal, nuclear) and transfers between them
- —Apply the principle of conservation of energy
- —Calculate kinetic energy, gravitational potential energy, work done and power
- —Define and calculate efficiency
- —Evaluate energy resources: fossil fuels, nuclear, solar, wind, hydroelectric, geothermal
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Principle of conservation of energy
- Energy cannot be created or destroyed; it is only transferred between stores. The total energy is constant.
- Work done
- The energy transferred when a force moves an object: W = F × d, where d is the distance moved in the direction of the force.
- Power
- The work done, or energy transferred, per unit time.
- Efficiency
- The fraction of the input energy that is usefully transferred: (useful output ÷ total input) × 100%.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Forgetting to square the velocity in kinetic energy — or squaring after multiplying by the mass. Substitute v², then multiply.
- 02
Using the slope length instead of the vertical height in GPE. The h in mgh is always the vertical height gained.
- 03
Writing 'energy is lost'. Energy is never lost — it is transferred to less useful stores, usually the internal (thermal) energy of the surroundings. That wording is worth the mark.
- 04
Giving efficiency greater than 100%, then not noticing. If your efficiency exceeds 100%, the useful and total values are swapped.
One worked example, done properly
Question
A 0.50 kg ball is dropped from a height of 20 m. Ignoring air resistance, calculate its speed just before it hits the ground. (g = 10 m/s²)
Method
- 1.Energy conservation: KE gained = GPE lost, so ½mv² = mgh.
- 2.The mass cancels: v² = 2gh = 2 × 10 × 20 = 400.
- 3.v = √400.
v = 20 m/s
Test yourself
10 questions · instant marking
Question 1 of 10
A 2 kg object moves at 3 m/s. Its kinetic energy is:
Question 2 of 10
A 4 kg mass is lifted 5 m (g = 10 m/s²). The gain in gravitational PE is:
Question 3 of 10
A motor transfers 3000 J in 60 s. Its power is:
Question 4 of 10
A lamp receives 200 J and usefully transfers 30 J as light. Its efficiency is:
Question 5 of 10
Energy that is 'wasted' by a machine is usually transferred to:
Question 6 of 10
A force of 50 N pushes a box 4 m in the direction of the force. Work done is:
Question 7 of 10
A ball is dropped and air resistance is negligible. Just before landing, its kinetic energy equals:
Question 8 of 10
Which energy resource does NOT originate from the Sun?
Question 9 of 10
Doubling the speed of an object multiplies its kinetic energy by:
Question 10 of 10
A crane lifts a 100 kg load 6 m in 12 s (g = 10 m/s²). Its useful output power is: