Curriculum fit: GCSE Physics and Combined Science (energy). US: middle and high school physical science (energy).
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What Is It?
Energy cannot be created or destroyed, only transferred between stores. This is the principle of conservation of energy. Some energy is usually transferred to the surroundings as wasted thermal energy.
Two important stores are kinetic energy (energy of movement) and gravitational potential energy (energy due to height).
Key Formulas and Ideas
- Kinetic energy = ½ × m × v².
- Gravitational potential energy = m × g × h (g ≈ 9.8 N/kg).
- Efficiency = useful energy output ÷ total energy input.
- Energy is measured in joules (J).
Worked Examples
Kinetic Energy
Find the kinetic energy of a 2 kg object moving at 3 m/s.
- KE = ½ × m × v².
- = ½ × 2 × 9 = 9.
✓ Answer 9 J
Potential Energy
Find the gravitational potential energy of a 2 kg object 5 m above the ground (g = 9.8 N/kg).
- GPE = m × g × h.
- = 2 × 9.8 × 5 = 98.
✓ Answer 98 J
Falling Object
A ball is dropped from 10 m. Ignoring air resistance, find its speed just before it lands (g = 9.8 N/kg).
- GPE lost equals KE gained: mgh = ½mv².
- v² = 2gh = 2 × 9.8 × 10 = 196.
- v = √196 = 14.
✓ Answer 14 m/s
Common Mistakes to Avoid
- Forgetting to square the velocity in kinetic energy.
- Forgetting the ½ in the kinetic energy formula.
- Using grams instead of kilograms.
- Saying that energy is 'used up' rather than transferred.
Practice Questions
Frequently Asked Questions
Why does kinetic energy depend on v squared?
Doubling the speed makes the kinetic energy four times larger, which is why high speeds are so much more dangerous in crashes.
Why does doubling speed quadruple kinetic energy?
Kinetic energy depends on v², so doubling v multiplies the energy by 2², which is 4.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.