- Define and explain Potential Energy in your own words
- Use key terms such as gravitational potential energy accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Potential Energy: working with gravitational potential energy, E = mgh, and stored elastic energy.
Working with gravitational potential energy, E = mgh, and stored elastic energy.
Key ideas
Energy is stored in different ways
A moving car holds kinetic energy, a raised weight holds gravitational potential energy, a stretched spring holds elastic potential energy, and food and fuels hold chemical energy. The unit is always the joule, and energy can move from one store to another — a battery's chemical store becomes electrical, then light.
Energy is conserved — never created or destroyed
In every change, the total energy stays the same; it only transfers between stores or dissipates as heat and sound. A falling ball's gravitational potential energy becomes kinetic energy. Sankey diagrams show these transfers with arrow widths matching the amounts, and the arrows always add up.
Key term — gravitational potential energy: Energy stored by lifting an object: E = mass × g × height.
A 1000 kg car moves at 20 m/s. Calculate its kinetic energy.
½ × 1000 × 20² = ½ × 1000 × 400 = 200,000 J (200 kJ).
Answer: ½ × 1000 × 20² = ½ × 1000 × 400 = 200,000 J (200 kJ).
- Saying energy is 'used up' Energy is never used up; it transfers to other stores, often dissipating as heat.
- Forgetting to square the velocity in kinetic energy KE = ½mv² — doubling the speed quadruples the energy, so the square matters enormously.
Practice
(400 ÷ 500) × 100% = 80%.
Chemical energy (battery) → electrical energy → light energy, plus some thermal energy in the bulb.
None is lost — some transfers to thermal energy and sound in the ball and ground on each bounce.
Kinetic energy depends on v², so doubling v multiplies the energy by 2² = 4.
Quick check
Which of these best defines "gravitational potential energy"?
A 3 kg book is lifted 2 m (g = 10 N/kg). Find its gain in GPE.
- Potential Energy: working with gravitational potential energy, E = mgh, and stored elastic energy.
- Energy is stored in different ways: A moving car holds kinetic energy, a raised weight holds gravitational potential energy, a stretched spring holds elastic potential energy, and food and fuels hold chemical energy.
- kinetic energy: Energy stored in a moving object: E = ½ × mass × velocity².
- Watch out for: saying energy is 'used up'