Unit 11: Work, Power and Energy Calculations
This unit brings together everything about energy: how to calculate work done, kinetic energy, gravitational potential energy, and power.
01 Work done
work done (J) = force (N) × distance moved in the direction of the force (m)
Work is done whenever a force causes something to move. Work done is measured in joules (J), the same unit as energy — because doing work is a way of transferring energy.
02 Kinetic and gravitational potential energy
kinetic energy (J) = ½ × mass (kg) × velocity² (m/s)²
gravitational potential energy (J) = mass (kg) × gravitational field strength (N/kg) × height (m)
On Earth, gravitational field strength is about 9.8 N/kg (often rounded to 10 N/kg).
03 Power
power (W) = energy transferred (J) ÷ time taken (s)
Power is the rate of doing work or transferring energy. A more powerful machine transfers the same energy in less time, or transfers more energy in the same time.
V Key vocabulary
| Term | Meaning |
|---|---|
| Work done | Energy transferred when a force moves an object, measured in joules. |
| Kinetic energy | Energy an object has because of its motion. |
| Gravitational potential energy | Energy an object has because of its height above the ground. |
| Power | The rate of transferring energy, measured in watts (W). |
? Quick check
Try to answer before you tap. Answers are hidden underneath.
A 1200 kg car travels at 20 m/s. Calculate its kinetic energy.
240,000 J (KE = ½ × 1200 kg × 20² = ½ × 1200 × 400 = 240,000 J).
A crane lifts a 500 kg load 8 m upward. Calculate the gravitational potential energy gained (use g = 10 N/kg).
40,000 J (GPE = m × g × h = 500 kg × 10 N/kg × 8 m = 40,000 J).
A motor transfers 6000 J of energy in 20 s. Calculate its power.
300 W (P = E ÷ t = 6000 J ÷ 20 s = 300 W).