OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Work and conservation of energy: (a) work done by a force; the unit joule
Work is done when a force moves its point of application through a displacement, and for a constant force in the direction of motion W = Fs, where W is work in joules, F is the force in newtons…
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Work is done when a force moves its point of application through a displacement, and for a constant force in the direction of motion W = Fs, where W is work in joules, F is the force in newtons and s is the displacement in metres. One joule is the work done when a force of one newton moves an object one metre in the direction of the force, so 1 J = 1 N m. If the force acts at an angle θ to the displacement, only the component along the displacement does work, W = Fs cos θ.
For example: A 20 N force pushing a box 3.0 m along its direction does W = 20 × 3.0 = 60 J. Work done on an object transfers energy to it, and work done by it transfers energy away.
What earns the marks
- States that work is done when a force moves its point of application through a displacement.
- Applies W = Fs for a constant force acting along the displacement.
- Defines the joule as the work done by a one-newton force moving an object one metre in the direction of the force, so 1 J = 1 N m.
- Uses W = Fs cos θ when the force is at an angle θ to the displacement.
- Links work done to energy transfer, recognising that work done on an object increases its energy store.
Common mistakes and how to put them right
Using the total force when it acts at an angle.
Put it right: The error is ignoring the direction, and the correction is to use the component along the displacement, F cos θ.
Treating work as a vector.
Put it right: The error is giving work a direction, and the correction is to recognise that work and energy are scalar quantities measured in joules.
Assuming a force does work even when there is no displacement.
Put it right: The error is forgetting the displacement condition, and the correction is to note that no work is done if the point of application does not move.
Exam tips
- Check that the force and displacement are along the same line; if not, resolve the force before multiplying.
- Write the unit as J and remember 1 J = 1 N m, which helps you check equations by unit analysis.
- For a force perpendicular to the motion, such as a centripetal force, state that no work is done because cos 90° = 0.
Try the lesson
The start of the Work and conservation of energy: (a) work done by a force; the unit joule lesson, made from checked teaching notes for this specification point.
Listen
Work is done when a force moves its point of application through a displacement, and for a constant force in the direction of motion W = Fs, where W is work in joules, F is the force in newtons and s is the displacement in metres. One joule is the work done when a force of one newton moves an object one metre in the direction of the force, so 1 J = 1 N m. If the force acts at an angle θ to the displacement, only the component along the displacement does work, W = Fs cos θ. For example, a 20 N force pushing a box 3.0 m along its direction does W = 20 × 3.0 = 60 J. Work done on an object transfers energy to it, and work done by it transfers energy away. A strong answer earns marks for these points. States that work is done when a force moves its point of application through a displacement. Applies W = Fs for a constant force acting along the displacement. Defines the joule as the work done by a one-newton force moving an object one metre in the direction of the force, so 1 J = 1 N m. Uses W = Fs cos θ when the force is at an angle θ to the displacement. Exam tip: Check that the force and displacement are along the same line; if not, resolve the force before multiplying.
See it
Key words
Say what each word means before you turn the card.
Work done by a force
Try answering aloud or in your head before turning the card.
The joule
Try answering aloud or in your head before turning the card.
Work done by a force at an angle to the displacement
Try answering aloud or in your head before turning the card.
Work as a transfer of energy
Try answering aloud or in your head before turning the card.
See every definition
- Work done by a force
- Work is done when a force moves its point of application through a displacement.
- The joule
- One joule is the work done when a force of one newton moves an object one metre in the direction of the force, so 1 J = 1 N m.
- Work done by a force at an angle to the displacement
- If the force acts at an angle θ to the displacement, only the component along the displacement does work, W = Fs cos θ.
- Work as a transfer of energy
- Work done on an object transfers energy to it, and work done by it transfers energy away.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Work and conservation of energy: (a) work done by a force; the unit joule
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge