SEAB Original-style practice: A worker lifts a 15\ kg box through a height of 2\ m in 5\ s. Take g = 10\ N/kg. (a) Calculate the work done. (b) Calculate the power.

(a) Lifting work done equals the gain in gravitational potential energy: W = mgh = 15 × 10 × 2 = 300\ J. (b) Power = work done divided by time: P = Wt = 3005 = 60\ W. What markers reward: using mgh for the work done against gravity, the joule as the unit of energy, and power as energy per second in watts.

SingaporeCombined ScienceSyllabus dot point

What do we mean by work, energy and power, and how is energy stored and transferred?

Describe the main stores of energy and the principle of conservation of energy, and calculate work done, kinetic energy, gravitational potential energy and power

A focused N(A)-Level answer on energy. Stores of energy and conservation, plus calculating work done, kinetic and gravitational potential energy, and power with simple numbers.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

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

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to name the main stores of energy, state that energy is conserved, and carry out simple calculations of work done, kinetic energy, gravitational potential energy and power. The central idea is that energy is never made or destroyed, only transferred from one store to another.

The answer

Stores of energy

Energy can be stored in several ways, including:

kinetic energy (in a moving object),
gravitational potential energy (in a raised object),
chemical energy (in food, fuel and batteries),
elastic energy (in a stretched or squashed spring),
thermal energy (in a hot object).

Energy is measured in joules ( $\text{J}$ ).

Conservation of energy

The principle of conservation of energy says energy cannot be created or destroyed, only transferred from one store to another or moved from place to place. The total energy always stays the same. Some energy is usually transferred to the surroundings as wasted thermal energy, which is why no machine is perfectly efficient.

Work done

Work is done when a force moves something through a distance:

\text{work done} = \text{force} \times \text{distance} \qquad W = Fd

Work is measured in joules, the same unit as energy, because doing work transfers energy.

Kinetic energy and gravitational potential energy

A moving object has kinetic energy:

E_k = \tfrac{1}{2}mv^2

A raised object has gravitational potential energy:

E_p = mgh

where $h$ is the height and $g$ is the gravitational field strength (about $10\ \text{N/kg}$ ).

Power

Power is the rate of transferring energy, or how much energy is transferred each second:

\text{power} = \frac{\text{energy transferred}}{\text{time}} \qquad P = \frac{E}{t}

Power is measured in watts ( $\text{W}$ ), where $1\ \text{W} = 1\ \text{J/s}$ .

Energy transfer down a slide

A child of mass $30\ \text{kg}$ slides down from a height of $3\ \text{m}$ . Take $g = 10\ \text{N/kg}$ . Find the gravitational potential energy at the top, and the speed at the bottom if all of it becomes kinetic energy.

Step 1: Find the gravitational potential energy at the top

E_p = mgh = 30 \times 10 \times 3 = 900\ \text{J}

Step 2: Apply conservation of energy

If all the potential energy becomes kinetic energy, then at the bottom:

E_k = 900\ \text{J}

Step 3: Write the kinetic energy formula

\tfrac{1}{2}mv^2 = 900

Step 4: Solve for the speed

\tfrac{1}{2} \times 30 \times v^2 = 900 \;\Rightarrow\; 15v^2 = 900 \;\Rightarrow\; v^2 = 60 \;\Rightarrow\; v = 7.7\ \text{m/s}

So the child reaches about $7.7\ \text{m/s}$ at the bottom (ignoring friction).

Examples in context

Example 1. A falling coconut. As a coconut falls from a tree, its gravitational potential energy is transferred to kinetic energy, so it speeds up as it drops. Just before it lands, almost all the potential energy has become kinetic energy, which is why a fall from greater height hits harder.

Example 2. Comparing two light bulbs. A $60\ \text{W}$ bulb transfers $60\ \text{J}$ of energy every second, while a $10\ \text{W}$ LED transfers only $10\ \text{J}$ each second for similar brightness. The lower power means less energy used over time, which is why LEDs cost less to run.

Try this

Cue. Calculate the work done when a $25\ \text{N}$ force pushes a box $4\ \text{m}$ : $W = 25 \times 4 = 100\ \text{J}$ .
Cue. Find the kinetic energy of a $2\ \text{kg}$ ball moving at $3\ \text{m/s}$ : $E_k = \tfrac{1}{2} \times 2 \times 3^2 = 9\ \text{J}$ .
Cue. A motor transfers $400\ \text{J}$ in $8\ \text{s}$ . Find its power: $P = \dfrac{400}{8} = 50\ \text{W}$ .

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA worker lifts a

15\ \text{kg}

box through a height of

2\ \text{m}

5\ \text{s}

. Take

g = 10\ \text{N/kg}

. (a) Calculate the work done. (b) Calculate the power.

Show worked answer →

(a) Lifting work done equals the gain in gravitational potential energy:

$W = mgh = 15 \times 10 \times 2 = 300\ \text{J}$ .

(b) Power = work done divided by time:

$P = \dfrac{W}{t} = \dfrac{300}{5} = 60\ \text{W}$ .

What markers reward: using $mgh$ for the work done against gravity, the joule as the unit of energy, and power as energy per second in watts.

Original3 marksA ball of mass

0.5\ \text{kg}

moves at

4\ \text{m/s}

. (a) Calculate its kinetic energy. (b) State the energy change as the ball is thrown straight up and slows down.

Show worked answer →

(a) Kinetic energy:

$E_k = \tfrac{1}{2}mv^2 = \tfrac{1}{2} \times 0.5 \times 4^2 = \tfrac{1}{2} \times 0.5 \times 16 = 4\ \text{J}$ .

(b) As the ball rises and slows, kinetic energy is transferred to gravitational potential energy.

What markers reward: squaring the speed before multiplying, the joule unit, and naming the transfer from kinetic to gravitational potential energy.