OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply: moment of a force (N m) = force (N) × distance (m)…
Full specification wording
Forces in action: recall and apply: moment of a force (N m) = force (N) × distance (m) (normal to direction of the force) (content found only in the physics separate science qualification)
A moment measures the turning effect of a force about a pivot.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 21
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A moment measures the turning effect of a force about a pivot. The equation is M = F × d, where F is the force in newtons and d is the perpendicular distance in metres from the pivot to the line of action of the force, measured normal to the force's direction. The moment is in newton metres (N m). A 20 N force applied 0.5 m from a pivot, perpendicular to the lever, gives M = 20 × 0.5 = 10 N m. If the force acts at an angle, use only the perpendicular distance, not the slant length along the lever. Doubling either the force or the perpendicular distance doubles the moment. Convert distances to metres first: 35 cm = 0.35 m. This content appears only in the physics separate science qualification.
What earns the marks
- Selects the relationship moment equals force multiplied by the perpendicular distance from the pivot.
- Identifies the distance as measured normal to the direction of the force, in metres.
- States the moment unit as the newton metre.
- Calculates a moment by multiplying force by perpendicular distance, for example 20 N × 0.5 m = 10 N m.
- Converts a distance given in centimetres to metres before substituting.
Common mistakes and how to put them right
Uses the slant length along a lever instead of the perpendicular distance.
Put it right: Measure normal to the force.
Substitutes a distance in centimetres without converting.
Put it right: Divide by 100 to reach metres.
Divides force by distance instead of multiplying.
Put it right: Multiply the force by the perpendicular distance.
Reports the moment in newtons.
Put it right: Give the answer in newton metres.
Exam tips
- Sketch the pivot and mark the perpendicular distance before substituting values.
- Convert every distance to metres at the start so the newton metre result follows directly.
- Check the answer by reasoning: doubling the perpendicular distance should double the moment.
Try the lesson
The start of the Forces in action: recall and apply: moment of a force (N m) = force (N) × distance (m) (normal to direction of the force) (content found only in the physics separate science qualification) lesson, made from checked teaching notes for this specification point.
Have a go first
A force of 8 N acts perpendicular to a lever at a distance of 0.5 m from the pivot. Calculate the moment of the force about the pivot.
Listen
A moment measures the turning effect of a force about a pivot. The equation is M = F × d, where F is the force in newtons and d is the perpendicular distance in metres from the pivot to the line of action of the force, measured normal to the force's direction. The moment is in newton metres (N m). A 20 N force applied 0.5 m from a pivot, perpendicular to the lever, gives M = 20 × 0.5 = 10 N m. If the force acts at an angle, use only the perpendicular distance, not the slant length along the lever. Doubling either the force or the perpendicular distance doubles the moment. Convert distances to metres first: 35 cm = 0.35 m. This content appears only in the physics separate science qualification. A strong answer earns marks for these points. Selects the relationship moment equals force multiplied by the perpendicular distance from the pivot. Identifies the distance as measured normal to the direction of the force, in metres. States the moment unit as the newton metre. Calculates a moment by multiplying force by perpendicular distance, for example 20 N × 0.5 m = 10 N m. Exam tip: Sketch the pivot and mark the perpendicular distance before substituting values.
See it
Put it in order
Put these steps in the order they happen.
- 1. Identify the perpendicular distance d from the pivot, normal to the force
- 2. Convert d to metres
- 3. Identify the force F in newtons
- 4. Moment in newton metres (N m)
- 5. Multiply: moment = F × d
Key words
Say what each word means before you turn the card.
Moment as the turning effect of a force
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Perpendicular distance from the pivot
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The newton metre as the unit of moment
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Unit conversion of distance to metres
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See every definition
- Moment as the turning effect of a force
- A moment measures the turning effect of a force about a pivot.
- Perpendicular distance from the pivot
- The distance from the pivot to the line of action of the force, measured normal to the force's direction, in metres.
- The newton metre as the unit of moment
- The moment is in newton metres (N m).
- Unit conversion of distance to metres
- Convert distances to metres first, for example 35 cm = 0.35 m.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
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