What is the principle of conservation of energy?

The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another or converted from one form to another. The total energy of an isolated system stays constant, so in a falling object gravitational potential energy is converted into kinetic energy, with the total remaining the same when air resistance is ignored.

How are kinetic energy and gravitational potential energy calculated?

Kinetic energy is $E_k = \tfrac{1}{2}mv^2$, where $m$ is mass and $v$ is speed, so it grows with the square of the speed. Gravitational potential energy is $E_p = mgh$, where $h$ is the height gained and $g$ the gravitational field strength. Both are measured in joules, and for a falling object the loss in $E_p$ equals the gain in $E_k$ when resistance is ignored.

What is work done in O-Level Physics?

Work done is the energy transferred when a force moves an object in the direction of the force, calculated as $W = F \times d$, where $F$ is the force and $d$ is the distance moved in the direction of the force. It is measured in joules, and one joule is the work done when a force of one newton moves an object one metre.

What is the difference between power and energy?

Energy is the capacity to do work, measured in joules, while power is the rate at which energy is transferred, measured in watts ($1\ \text{W} = 1\ \text{J s}^{-1}$). Power is found from $P = E/t$, so two machines can transfer the same energy, but the more powerful one does it in less time.

How is efficiency calculated and why is it never above 100 percent?

Efficiency is the useful energy output divided by the total energy input, often expressed as a percentage: efficiency equals useful output over total input times one hundred. It can never exceed one hundred percent because energy is conserved, so the useful output can never be more than the input; some energy is always wasted, usually as heat or sound.

SingaporePhysics

Energy, Work and Power for Singapore O-Level Physics (6091): energy stores and transfers, kinetic and gravitational potential energy, work done by a force, and power and efficiency

A Singapore O-Level Physics (SEAB 6091) overview of Energy, Work and Power. It covers energy stores and the principle of conservation of energy, the formulae for kinetic and gravitational potential energy, the work done by a force, and the definitions of power and efficiency with worked calculations.

Generated by Claude Opus 4.87 min readSEAB-6091Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module covers
Energy stores and transfers
Kinetic and potential energy
Work done
Power and efficiency
How this module is examined
Check your knowledge

What this module covers

Energy, Work and Power gives O-Level Physics (SEAB 6091) one of its most powerful tools: the idea that energy is conserved as it moves between stores. The module catalogues the main energy stores, quantifies the two mechanical forms (kinetic and gravitational potential energy), defines work done as energy transferred by a force, and introduces power and efficiency for comparing how quickly and how usefully energy is transferred.

The conservation principle ties this module to mechanics (a falling object swaps potential for kinetic energy) and to electricity and thermal physics later. Each dot point below has full worked answers and practice questions.

Energy stores and transfers

Energy stores and transfers lists the major energy stores, including kinetic, gravitational potential, elastic potential, chemical, thermal (internal), electrical and nuclear, and the ways energy is transferred between them. The central rule is the principle of conservation of energy: energy is never created or destroyed, only moved or converted.

Seeing a process as a chain of transfers (chemical to kinetic to thermal, for example) is the reasoning examiners reward.

Kinetic and potential energy

Kinetic and potential energy gives the two mechanical formulae:

E_k = \tfrac{1}{2}mv^2, \qquad E_p = mgh.

Because

E_k

depends on the square of the speed, doubling the speed quadruples the kinetic energy. For a falling object, the loss in

E_p

equals the gain in

E_k

when air resistance is ignored.

Work done

Work done defines work as the energy transferred when a force moves its point of application,

W = F \times d,

measured in joules. No work is done by a force if there is no movement in its direction, which is why holding a heavy bag still does no mechanical work even though it feels tiring.

Power and efficiency

Power and efficiency defines power as the rate of energy transfer,

P = \frac{E}{t},

measured in watts, and efficiency as

\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\%.

Efficiency is always below

100\%

because some energy is transferred to less useful stores such as heat and sound.

How this module is examined

Use conservation of energy. Many problems are solved fastest by equating energy before and after, such as $E_p$ lost equals $E_k$ gained.
Mind the square in $E_k$ . Remember that kinetic energy depends on $v^2$ , so speed changes have a large effect.
Separate power from energy. Power is energy per second; do not quote a power in joules or an energy in watts.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

State the principle of conservation of energy. (2 marks)
A $2.0\ \text{kg}$ object moves at $3.0\ \text{m s}^{-1}$ . Calculate its kinetic energy. (2 marks)
A force of $25\ \text{N}$ moves a crate $4.0\ \text{m}$ in the direction of the force. Calculate the work done. (2 marks)
A motor transfers $600\ \text{J}$ of energy in $5.0\ \text{s}$ . Calculate its power. (2 marks)
A lamp takes in $60\ \text{J}$ of electrical energy and gives out $9\ \text{J}$ of light. Calculate its efficiency. (2 marks)

Sources & how we know this

Singapore-Cambridge GCE O-Level Physics (Syllabus 6091) — Singapore Examinations and Assessment Board (2026)