AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Power: work done, W, in joules, J
Work done measures the energy transferred when a force moves an object along its line of action.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Work done measures the energy transferred when a force moves an object along its line of action. The equation is W = F s, where W is work done in joules (J), F is force in newtons (N) and s is distance moved in metres (m). One joule equals one newton metre, so lifting a 2 N apple 0.5 m transfers W = 2 × 0.5 = 1 J. Work is a scalar; only the component of force along the displacement does work. If force and motion are perpendicular, no work is done. Energy transferred and work done are numerically equal, so the joule also measures energy. In power calculations, divide work done by time taken, giving P = W ÷ t. Always convert centimetres to metres and grams to kilograms before substituting, and check the unit of every quantity.
What earns the marks
- States that work done is the energy transferred when a force moves an object through a distance along the force's line of action.
- Recalls and applies W = F s, substituting force in newtons and distance in metres to give work done in joules.
- Recognises that 1 J = 1 N m and that work done and energy transferred are numerically equal.
- Uses work done in power calculations by dividing by time, for example P = W ÷ t.
- Handles unit conversions correctly, such as 50 cm = 0.50 m, before calculating.
- Explains that a force perpendicular to the displacement does no work on the object.
Common mistakes and how to put them right
Using the distance travelled rather than the distance moved along the line of the force.
Put it right by resolving the force or using the component of displacement parallel to the force.
Forgetting to convert centimetres to metres, giving an answer 100 times too large.
Put it right by dividing centimetres by 100 before substituting.
Confusing work done in joules with power in watts.
Put it right by remembering work is energy transferred, while power is the rate of transfer, P = W ÷ t.
Exam tips
- Write the equation, substitute numbers with units, then give the answer with the correct unit, for example W = 15 N × 3 m = 45 J.
- Check whether the question asks for work done or power; if it gives a time, you may need to divide by time.
- Show conversion steps clearly, such as 250 g = 0.25 kg, so the examiner can award method credit even if the final value slips.
Try the lesson
The start of the Power: work done, W, in joules, J lesson, made from checked teaching notes for this specification point.
Have a go first
A motor has a power output of 50 W. How much work is done by the motor in 60 seconds?
Listen
Work done measures the energy transferred when a force moves an object along its line of action. The equation is W = F s, where W is work done in joules (J), F is force in newtons (N) and s is distance moved in metres (m). One joule equals one newton metre, so lifting a 2 N apple 0.5 m transfers W = 2 × 0.5 = 1 J. Work is a scalar; only the component of force along the displacement does work. If force and motion are perpendicular, no work is done. Energy transferred and work done are numerically equal, so the joule also measures energy. In power calculations, divide work done by time taken, giving P = W ÷ t. Always convert centimetres to metres and grams to kilograms before substituting, and check the unit of every quantity. A strong answer earns marks for these points. States that work done is the energy transferred when a force moves an object through a distance along the force's line of action. Recalls and applies W = F s, substituting force in newtons and distance in metres to give work done in joules. Recognises that 1 J = 1 N m and that work done and energy transferred are numerically equal. Uses work done in power calculations by dividing by time, for example P = W ÷ t. A common slip: Using the distance travelled rather than the distance moved along the line of the force. Instead, resolving the force or using the component of displacement parallel to the force. Exam tip: Write the equation, substitute numbers with units, then give the answer with the correct unit, for example W = 15 N × 3 m = 45 J.
Key words
Say what each word means before you turn the card.
Work done
Try answering aloud or in your head before turning the card.
Joule
Try answering aloud or in your head before turning the card.
Power
Try answering aloud or in your head before turning the card.
See every definition
- Work done
- The energy transferred when a force moves an object through a distance along the force's line of action, measured in joules, J.
- Joule
- One newton metre; it measures work done and energy transferred, which are numerically equal.
- Power
- Calculated by dividing work done by time taken, giving P = W ÷ t.
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