OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Define and calculate the moment of a force (to include: application of the…
Full specification wording
Forces in action: define and calculate the moment of a force (to include: application of the principle of moments for objects which are balanced) (content found only in the physics separate science qualification)
The moment of a force measures its turning effect about a pivot.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 18 of 21
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The moment of a force measures its turning effect about a pivot. Define it as the force multiplied by the perpendicular distance from the pivot to the line of action of the force, so moment = force × perpendicular distance, with the force in newtons and the distance in metres giving the moment in newton-metres (N m). To calculate, identify the pivot, find the perpendicular distance, then multiply. For a balanced object the principle of moments applies: the total clockwise moment about the pivot equals the total anticlockwise moment.
For example: A 20 N force at 0.5 m gives 10 N m; if this balances an unknown force at 0.25 m on the other side, that force is 10 N m ÷ 0.25 m = 40 N.
What earns the marks
- Defines the moment as force multiplied by the perpendicular distance from the pivot to the line of action of the force.
- Calculates a moment by multiplying force in newtons by perpendicular distance in metres and gives the unit as newton-metres.
- Applies the principle of moments to a balanced object by equating total clockwise and total anticlockwise moments about the pivot.
- Rearranges the moment equation to find an unknown force or distance and substitutes values correctly.
Common mistakes and how to put them right
Uses the distance along the object rather than the perpendicular distance from the pivot.
Put it right: Measure perpendicular to the line of action.
Adds moments on opposite sides instead of equating them.
Put it right: For balance, set total clockwise moment equal to total anticlockwise moment.
Mixes units such as centimetres with newtons without converting.
Put it right: Convert distances to metres before multiplying.
Exam tips
- Write the equation moment = force × perpendicular distance before substituting numbers.
- Convert all distances to metres and check the final unit is newton-metres.
- For balance questions, label clockwise and anticlockwise moments separately, then set them equal.
Try the lesson
The start of the Forces in action: define and calculate the moment of a force (to include: application of the principle of moments for objects which are balanced) (content found only in the physics separate science qualification) lesson, made from checked teaching notes for this specification point.
Have a go first
A spanner is used to turn a nut. A force of 40 N is applied at a perpendicular distance of 0.25 m from the centre of the nut. What is the moment of the force about the nut?
Listen
The moment of a force measures its turning effect about a pivot. Define it as the force multiplied by the perpendicular distance from the pivot to the line of action of the force, so moment = force × perpendicular distance, with the force in newtons and the distance in metres giving the moment in newton-metres (N m). To calculate, identify the pivot, find the perpendicular distance, then multiply. For a balanced object the principle of moments applies: the total clockwise moment about the pivot equals the total anticlockwise moment. For example, a 20 N force at 0.5 m gives 10 N m; if this balances an unknown force at 0.25 m on the other side, that force is 10 N m ÷ 0.25 m = 40 N. A strong answer earns marks for these points. Defines the moment as force multiplied by the perpendicular distance from the pivot to the line of action of the force. Calculates a moment by multiplying force in newtons by perpendicular distance in metres and gives the unit as newton-metres. Applies the principle of moments to a balanced object by equating total clockwise and total anticlockwise moments about the pivot. Rearranges the moment equation to find an unknown force or distance and substitutes values correctly. Exam tip: Write the equation moment = force × perpendicular distance before substituting numbers.
See it
Put it in order
Put these steps in the order they happen.
- 1. Substitute values correctly
- 2. Rearrange the moment equation to find an unknown force or distance
- 3. moment = force × perpendicular distance
- 4. Balanced object: total clockwise moment equals total anticlockwise moment
- 5. Identify the pivot
- 6. Find the perpendicular distance from the pivot to the line of action of the force
- 7. Force in newtons, distance in metres, moment in newton-metres (N m)
Key words
Say what each word means before you turn the card.
Moment
Try answering aloud or in your head before turning the card.
Principle of moments
Try answering aloud or in your head before turning the card.
Balanced object
Try answering aloud or in your head before turning the card.
See every definition
- Moment
- The force multiplied by the perpendicular distance from the pivot to the line of action of the force.
- Principle of moments
- For a balanced object, the total clockwise moment about the pivot equals the total anticlockwise moment.
- Balanced object
- An object for which the total clockwise moment about the pivot equals the total anticlockwise moment.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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