Energy, Work and Power

This guide covers the energy, work and power parts of subtopic 1.7 Energy, work and power, from Topic 1, Motion, forces and energy, for Cambridge O Level Physics 5054, 2026–2028 series. Energy resources and efficiency, the remaining part of 1.7, are covered separately in Energy Resources and Efficiency.

Where this fits in 5054

Work is what forces (from Forces and Motion) do when they move something, and it’s the mechanism by which energy is transferred from one store to another. Power then measures how quickly that transfer happens. Together these three ideas — energy, work and power — form the calculation core of Topic 1’s final section.

Syllabus coverage

CAMBRIDGE O LEVEL PHYSICS 5054

• State that energy may be stored as kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal) (1.7)
• Describe how energy is transferred between stores during events and processes, including transfer by forces (mechanical work), electrical currents (electrical work), heating, and electromagnetic, sound and other waves (1.7)
• Know the principle of the conservation of energy, and apply it to the transfer of energy between stores during events and processes (1.7)
• Recall and use the equation for kinetic energy, Ek = ½mv² (1.7)
• Recall and use the equation for the change in gravitational potential energy, ΔEp = mgΔh (1.7)
• Recall and use work done = force × distance moved in the direction of the force, W = Fd (1.7)
• Define power as work done per unit time and also as energy transferred per unit time; recall and use power = work done ÷ time taken and power = energy transferred ÷ time taken (1.7)

5054 is not tiered — every candidate covers all of the above.

Energy stores and transfers

Physics at this level treats energy as something held in named stores: kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal). An event or process transfers energy from one store to another, and the syllabus names the mechanisms of transfer: by a force doing mechanical work, by an electrical current doing electrical work, by heating, and by electromagnetic, sound or other waves.

The principle of conservation of energy underlies every such transfer: energy is never created or destroyed, only transferred between stores — the total amount stays constant. A falling ball transfers energy from its gravitational potential store to its kinetic store (and eventually, on landing, to internal/thermal and sound stores); a battery-powered motor transfers energy from a chemical store to a kinetic store via electrical work.

Kinetic energy

Kinetic energy is the energy an object has because of its motion:

Ek = ½mv²

Worked example. Find the kinetic energy of a 1500 kg car moving at 20 m/s.

Ek = ½mv² = ½ × 1500 × 20² = ½ × 1500 × 400 = 300 000 J

Because velocity is squared, doubling an object’s speed quadruples its kinetic energy — a small increase in speed has a disproportionately large effect on the energy involved, which is part of why braking distance increases sharply with speed (from Forces and Motion).

Gravitational potential energy

The change in gravitational potential energy when an object’s height changes:

ΔEp = mgΔh

where g is the gravitational field strength (≈ 9.8 N/kg, numerically the same value as the acceleration of free fall from Kinematics and Motion Graphs).

Worked example. Find the change in gravitational potential energy when a 5 kg object is lifted 3 m.

ΔEp = mgΔh = 5 × 9.8 × 3 = 147 J

Work done

Work is done whenever a force moves something in the direction of the force:

work done = force × distance moved in the direction of the force        W = Fd

Worked example. A force of 40 N pushes a crate 6 m across a floor. Find the work done.

W = Fd = 40 × 6 = 240 J

Work done and energy transferred are the same quantity measured the same way — doing work on an object is precisely how energy is transferred into or out of it by a force, which is why both are measured in joules.

Power

Power is the rate of energy transfer or the rate of doing work — how quickly energy moves from one store to another:

power = work done / time taken        P = W / t
power = energy transferred / time taken        P = ΔE / t

Worked example. A motor does 6000 J of work in 15 s. Find its power.

P = W / t = 6000 / 15 = 400 W

Common mistakes

• Forgetting to square the velocity in Ek = ½mv². A common error is calculating ½mv instead — always square v before multiplying.
• Using the wrong height in ΔEp = mgΔh. It’s the change in height that matters, not the absolute height above some fixed reference, unless the question specifically defines a reference level.
• Treating “work done” as different from “energy transferred.” They are the same thing, expressed the same way, in the same units (joules) — work is simply the name for energy transfer that happens via a force.
• Using the distance the object moves overall, rather than the distance moved in the direction of the force, when a force acts at an angle to the motion (this syllabus mostly restricts to forces and motion in the same direction, but the distinction is worth knowing).
• Mixing up power’s two equivalent formulas. P = W/t and P = ΔE/t give the same answer because work done and energy transferred are the same quantity — use whichever the question’s given values match.

Quick revision checklist

• The seven named energy stores, and the four transfer mechanisms (forces, electric currents, heating, waves)
• The principle of conservation of energy
• Ek = ½mv² — and why doubling speed quadruples kinetic energy
• ΔEp = mgΔh
• W = Fd
• P = W/t = ΔE/t

Related resources

• Energy Resources and Efficiency — the rest of subtopic 1.7
• Forces and Motion — the forces that do the work described here
• Cambridge O Level Physics subject hub

Written against Cambridge O Level Physics 5054, 2026–2028 series. Always check the current syllabus for your examination year.