Work done and energy transferred revision notes

Edexcel International GCSE Physics (4PH1), chapter 16: Work and power, specification point 4.11, 4.12. Free lesson.

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Work is done when a force moves an object in the direction of the force. Work done measures the energy transferred, so it is useful for calculating how much energy moves between stores.

How to calculate work done

Use the equation W = F × d. The distance must be how far the object moves in the direction of the force. A force of 1 N moving an object 1 m does 1 J of work, so a joule is also a newton metre (N m). W is work done, measured in joules (J). F is force, measured in newtons (N). d is distance moved in the direction of the force, measured in metres (m).

Work done is energy transferred

Doing work is one way to transfer energy. The work done is equal to the energy transferred, and both are measured in joules. When a student pushes a box, energy transfers from the student's chemical store. Most of it ends up in the thermal store of the box and the floor. Work done = energy transferred.

Work out the force when lifting

When you lift an object steadily, the lifting force equals its weight. Calculate the weight first using F = m × g, then use that force in the work equation. m is mass in kilograms (kg). g is gravitational field strength in newtons per kilogram (N/kg).

When no work is done

An object must move in the direction of the force for work to be done. If it does not move, the distance is zero, so the work done on it is zero. Your arms may still tire while holding a heavy bag, because your muscles use energy inside your body. No movement in the direction of the force means no work is done on the object.

Use metres and track the energy

Always convert the distance into metres before using W = F × d. For a braking car, the brakes do work as the car moves against their force. The energy transferred comes from the car's kinetic store and moves to the thermal store of the brakes, then to the surroundings. For example, 80 cm = 0.8 m, not 80 m.