Work Done and Power: KS3 Physics Explained

In physics, work is done whenever a force causes an object to move in the direction of that force. Power measures how quickly that work is done. Both ideas link force, distance, and time into a single framework that predicts the performance of everything from a weightlifter to a car engine.

What is work done in physics?

Work done is the energy transferred when a force moves an object through a distance in the direction of the force. The formula is:

Work done (J) = Force (N) × Distance (m)

Or in symbols: W = F × d

The unit of work done is the joule (J). One joule is the work done when a force of 1 newton moves an object 1 metre in the direction of the force.

Important: if a force acts but nothing moves — for example, you push hard against a wall that does not budge — no work is done. Effort without movement transfers no energy in the physics sense.

Also, if the motion is at right angles to the force — for example, carrying a heavy bag horizontally (the weight acts downwards, the motion is horizontal) — the force does no work on the horizontal motion. Only the component of force in the direction of motion counts.

How do you calculate work done?

Worked example 1: A student pushes a trolley with a force of 50 N over a distance of 4 m.

Work done = 50 N × 4 m = 200 J

Worked example 2: A crane lifts a crate of mass 200 kg through a height of 5 m. (Use g = 10 N/kg, so weight = 200 × 10 = 2000 N.)

Work done = 2000 N × 5 m = 10,000 J = 10 kJ

The work done against gravity is stored as gravitational potential energy (GPE) in the crate. This shows the deep link between work and energy: work done on a system equals the energy transferred to it.

What is power?

Power is the rate at which work is done — how much energy is transferred per second.

Power (W) = Work done (J) ÷ Time (s)

Or in symbols: P = W ÷ t

The unit of power is the watt (W), named after James Watt. One watt equals one joule per second (1 W = 1 J/s).

Because work done equals energy transferred, power can also be written as:

Power (W) = Energy transferred (J) ÷ Time (s)

These are equivalent statements. A 100 W light bulb transfers 100 J of energy every second. A 1 kW (1000 W) kettle transfers 1000 J every second.

How do you calculate power?

Quantity | Symbol | Unit
Power | P | Watt (W)
Work done / Energy transferred | W or E | Joule (J)
Time | t | Second (s)

Worked example 1 — stair climbing: A student of mass 50 kg runs up a flight of stairs 3 m high in 6 s. (Weight = 50 × 10 = 500 N.)

Work done = 500 N × 3 m = 1500 J

Power = 1500 J ÷ 6 s = 250 W

Worked example 2 — electric motor: A motor transfers 18,000 J of energy in 60 s.

Power = 18,000 ÷ 60 = 300 W

Worked example 3 — rearranging: A 2 kW (2000 W) hairdryer runs for 3 minutes (180 s). How much energy does it transfer?

Energy = Power × Time = 2000 × 180 = 360,000 J = 360 kJ

What is the difference between work, energy and power?

A frequent point of confusion at KS3:

Concept | Definition | Unit | Key point
Energy | The capacity to do work; stored in various forms | Joule (J) | Can be stored or transferred
Work done | Energy transferred by a force through a distance | Joule (J) | Requires force AND movement
Power | Rate of doing work / transferring energy | Watt (W) | Does NOT tell you how much energy — tells you how fast

Two climbers who scale the same cliff to the same height do the same work (same weight × same height). But if one reaches the top in half the time, they developed twice the power. Power depends on how quickly you act, not just how much you do.

How do work and power relate to machines and efficiency?

Machines — from bicycle gears to cranes — cannot reduce the total work needed (energy in = energy out, conservation of energy), but they can change the force and distance over which it is applied, or change the rate at which work is done.

Efficiency measures what fraction of the energy input is usefully transferred:

Efficiency = Useful energy output ÷ Total energy input (expressed as a decimal or percentage)

A motor that puts in 500 J and delivers 400 J of useful work has an efficiency of 400 ÷ 500 = 0.8 (80%). The remaining 20% (100 J) is wasted, usually as heat due to friction.

Higher-power machines are not automatically more efficient — a sports car may have a much more powerful engine than a small hatchback, yet be less fuel-efficient because it wastes more energy as heat and noise.

Frequently asked questions

What is work done in KS3 physics?

Work done in physics is the energy transferred when a force moves an object through a distance in the direction of that force. It is calculated as work done = force × distance, measured in joules. If there is no movement, or if the movement is at right angles to the force, no work is done in the physics sense.

What is the unit of power and what does it mean?

The unit of power is the watt (W), named after the Scottish engineer James Watt. One watt means one joule of energy is transferred every second. Larger units include the kilowatt (kW = 1000 W) and megawatt (MW = 1,000,000 W), used for household appliances and power stations respectively.

How is power different from energy?

Energy is the total amount of work that can be done — it is stored in fuels, food, or moving objects. Power is the rate at which energy is transferred or work is done. A very powerful machine can transfer a large amount of energy quickly; a low-power machine does the same total work but takes longer to do it.

Why does carrying a bag not count as doing work on it?

If you carry a bag horizontally at a constant height, the supporting force you exert (upwards) is at right angles to the direction of motion (horizontal). Work = force × distance in the direction of the force — and the perpendicular component is zero, so no work is done against gravity during horizontal carrying. Work is done only when the bag is lifted vertically.

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