Energy is the single idea that links Dynamics, Thermal Physics and Current Electricity. Almost every later topic asks you to track where energy goes. At O-Level, examiners expect the modern stores-and-transfers language. Answers written with the older “forms of energy” phrasing, such as “light energy”, “sound energy” or “heat energy”, lose marks.
This topic also covers how to calculate kinetic energy, gravitational potential energy, work done, power and efficiency. You will apply the Principle of Conservation of Energy and compare energy resources on four criteria: efficiency, cost, reliability and environmental impact.
What these notes cover
- •Energy stores and transfer pathways
- •The exam-standard sentence pattern
- •Kinetic energy
- •Gravitational potential energy
- •Conservation of energy: pendulum, roller-coaster and falling objects
- •Work done
- •Power
- •Efficiency
- •Renewable and non-renewable energy resources
- •The Singapore energy context
- •Writing a balanced discussion answer
- •Common mistakes and exam technique
Key definitions
- Energy store
- Where energy is kept in a system. A store can be filled or emptied, and the amount of energy in it is measured in joules (J).
- Transfer pathway
- The mechanism by which energy moves from one store to another. A pathway is a process, not a place, so energy never stays in a pathway.
- Internal store
- The total kinetic and potential energy of all the particles inside a substance. A hotter object has a larger internal store.
- Work done
- Work is done whenever a force moves an object in the direction of the force. The amount of work done equals the amount of energy transferred.
- Power
- The rate of energy transfer, or the rate of doing work. It is measured in watts (W), where 1 W = 1 J/s.
Key formulas
Kinetic energy
Eₖ = ½mv²
Eₖ is the kinetic energy in J, m is the mass in kg and v is the speed in m/s. Speed is squared, so doubling v makes Eₖ four times larger.
Gravitational potential energy
Eₚ = mgh
m is the mass in kg. g is the gravitational field strength; take g = 10 N/kg unless told otherwise. h is the vertical height gained in m, not the distance along a slope.
Conservation of energy (no resistive forces)
mgh₁ + ½mv₁² = mgh₂ + ½mv₂²
This applies at any two points 1 and 2 of a motion when no friction or air resistance acts. If resistive forces act, use: initial (Eₚ + Eₖ) = final (Eₚ + Eₖ) + energy transferred to the internal store.
Work done
W = Fd
F is the force in N and d is the distance moved in the direction of the force, in m. W is in joules, and 1 J = 1 N m. It is also written as W = Fs.
Power
P = E ÷ t = W ÷ t
E is the energy transferred in J, t is the time in s and P is in watts (W). Rearrange to E = Pt to find the total energy transferred.
Efficiency
efficiency = (useful energy output ÷ total energy input) × 100%
This also works with useful power output ÷ total power input. Efficiency has no unit, and it is always below 100% for a real device.
Worked example
A car of mass 1200 kg accelerates from 8.0 m/s to 20 m/s. Calculate the increase in its kinetic energy.
- 1Find the kinetic energy at the initial speed: Eₖ = ½mv² = ½ × 1200 × (8.0)² = 38400 J.
- 2Find the kinetic energy at the final speed: Eₖ = ½ × 1200 × (20)² = 240000 J.
- 3Subtract the two values: increase in Eₖ = 240000 − 38400 = 201600 J ≈ 2.0 × 10⁵ J.
Watch out: Do not use ½m(v − u)² = ½ × 1200 × (20 − 8.0)² = 86400 J. Because (v − u)² ≠ v² − u², always work out Eₖ at each speed separately and then subtract.
Common mistakes
Forgetting to square v in Eₖ = ½mv²
Wrong: For a 2 kg trolley moving at 6 m/s: Eₖ = ½ × 2 × 6 = 6 J.
Right: Eₖ = ½mv² = ½ × 2 × 6² = 36 J. Doubling the speed makes the kinetic energy four times larger, not twice as large.
Using the slope length instead of the vertical height
Wrong: A 40 kg crate is dragged 8.0 m up a ramp that rises 1.5 m: Eₚ = 40 × 10 × 8.0 = 3200 J.
Right: Eₚ = mgh = 40 × 10 × 1.5 = 600 J. Here h is the vertical height gained, never the distance travelled along the slope.
Saying energy is “lost” or “used up”
Wrong: The pendulum eventually stops because its energy is used up by friction.
Right: Work done against air resistance transfers energy from the kinetic energy store of the pendulum to the internal (thermal) energy store of the surrounding air. The total energy is unchanged, but the dissipated energy is no longer useful, so the swings become smaller.
Questions students ask
What are the six energy stores in Sec 3 Physics?
The six stores are kinetic, gravitational potential, chemical potential, elastic potential, nuclear and internal. Energy in a store is measured in joules (J). It moves between stores along four pathways: mechanically, electrically, by heating and by waves.
Why is “light energy” wrong in O-Level Physics?
Light is not a store. It is an electromagnetic wave, so it is a transfer by waves. Write that energy is transferred from the internal store of the lamp filament to the surroundings by light, never that light energy is produced; sound is likewise a transfer by mechanical waves.
Is work done when you push a wall that does not move?
No. In W = Fd, d is the distance moved in the direction of the force. The wall does not move, so d = 0 and W = 0 J, no matter how hard or how long you push. Time does not appear in the formula at all.
Why does Singapore use natural gas to generate electricity?
About 95% of Singapore’s electricity comes from imported natural gas, because it is the cleanest and most efficient fossil fuel and is highly reliable. Solar is the only viable renewable, but land scarcity limits how many panels can be installed. Singapore has no hydro or geothermal potential, and its tidal range and wind speeds are too small.
In the full PDF
- •Diagrams and figures, including energy flow diagrams, the kinetic energy against speed graph, the slope diagram and the pendulum at three positions
- •Further worked examples on describing energy transfers, gravitational potential energy, conservation of energy, and stair-climbing power and efficiency
- •Full comparison tables for coal, oil, natural gas, nuclear, solar, wind, hydroelectric, tidal, geothermal and biofuel
- •Model answers for 4-mark and 5-mark energy-resource discussion questions
- •The remaining common mistakes and exam technique, including SI unit conversions and the formula → substitution → answer habit
- •Section summaries and a final revision checklist for the whole topic