- Define and explain Kinetic Energy in your own words
- Use key terms such as kinetic energy accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
Energy is the currency of the universe — it pays for every movement, every light bulb and every thought. It cannot be created or destroyed, only moved around. This chapter shows you how to track it and calculate it.
This lesson focuses on Kinetic Energy: calculating the energy of moving objects with E = ½mv².
Calculating the energy of moving objects with E = ½mv².
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 — kinetic energy: Energy stored in a moving object: E = ½ × mass × velocity².
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).
- Forgetting to square the velocity in kinetic energy KE = ½mv² — doubling the speed quadruples the energy, so the square matters enormously.
- Saying energy is 'used up' Energy is never used up; it transfers to other stores, often dissipating as heat.
Practice
½ × 2 × 3² = 9 J.
Kinetic energy depends on v², so doubling v multiplies the energy by 2² = 4.
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.
Quick check
Which of these best defines "kinetic energy"?
A 3 kg book is lifted 2 m (g = 10 N/kg). Find its gain in GPE.
- Kinetic Energy: calculating the energy of moving objects with E = ½mv².
- 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.
- gravitational potential energy: Energy stored by lifting an object: E = mass × g × height.
- Watch out for: forgetting to square the velocity in kinetic energy