Work done and energy transfer: the work done equation, the link between work done and energy transferred, and how work done by friction raises temperature.

What this dot point is asking

Edexcel statements 8.5 to 8.7 want you to describe how to measure the work done by a force, to recall and use the work done equation, to understand that the energy transferred equals the work done, and to describe and calculate the energy changes when a system is changed by work done by forces, including the heating effect of friction.

The work done equation

The distance must be measured in the direction of the force: if a force acts horizontally, only the horizontal distance counts. Because work done equals energy transferred, the same value in joules describes both, which is why the equation links the mechanics of forces to the energy ideas of Topic 3.

The joule

Defining the joule as a newton metre links force, distance and energy in one statement. It also gives a quick unit check: if you multiply a force in newtons by a distance in metres you must get an energy in joules, so an answer in any other unit signals a mistake.

Work done against friction

Friction always opposes motion, so to keep an object moving against friction you must keep doing work, and that work ends up as thermal energy. This is why rubbing your hands together warms them, and why machine parts heat up. The energy is not lost; it is transferred to the thermal store, an idea that connects to dissipation and efficiency.

How Edexcel examines this

This dot point is examined on both tiers, mainly as a work-done calculation worth two or three marks and a short explanation linking work to energy or friction to heating. The mark scheme rewards the correct equation, substitution and unit, so write $E = Fd$ first and quote joules. A frequent twist is to give a force at an angle or to provide an irrelevant distance, testing whether you use only the distance in the direction of the force; for GCSE this usually means a horizontal force with a horizontal distance, or a vertical lift with a height. Explanation questions ask what happens to the energy when work is done against friction, where the full-mark answer states that the energy is transferred to the thermal store of the surfaces, which warm up, and that this equals the work done against friction. Examiners reward connecting work done to energy transferred (the same joules), and penalise saying energy is "lost" rather than transferred. Linking lifting work to a gain in gravitational potential energy, or stopping work to a loss of kinetic energy, strengthens an answer and connects to Topic 3.

Try this

Q1. State the work done equation. [1 mark]

• Cue. $E = F \times d$ (work done = force times distance in the direction of the force).

Q2. A force of $20\,\text{N}$ moves an object $5\,\text{m}$ in the direction of the force. Calculate the work done. [2 marks]

• Cue. $E = Fd = 20 \times 5 = 100\,\text{J}$.