OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Equilibrium: (a) moment of force; moment = Fx
The moment of a force is the turning effect of that force about a pivot.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The moment of a force is the turning effect of that force about a pivot. It is calculated as the product of the force and the perpendicular distance from the pivot to the line of action of the force: moment = Fx, where F is the force in newtons and x is the perpendicular distance in metres. The unit is the newton metre (N m). A larger force or a larger perpendicular distance gives a larger moment. If the force is not perpendicular to the lever, use the perpendicular component of the force or the perpendicular distance to the line of action. Moments are vector quantities in the sense that clockwise and anticlockwise moments oppose each other.
What earns the marks
- Moment of a force is the turning effect about a pivot.
- Moment = Fx, where F is force and x is perpendicular distance from pivot to line of action.
- The unit of moment is the newton metre (N m).
- Increasing force or perpendicular distance increases the moment.
- Clockwise and anticlockwise moments are treated as opposing when applying equilibrium.
Common mistakes and how to put them right
Using the distance along the lever rather than the perpendicular distance to the line of action.
Forgetting to convert distances from centimetres to metres before calculating.
Adding moments in the same direction instead of recognising clockwise and anticlockwise opposition.
Using the wrong unit, such as N, instead of N m.
Exam tips
- Identify the pivot and draw the perpendicular distance before substituting values.
- Convert all lengths to metres before using moment = Fx.
- Check the direction of each moment and state clockwise or anticlockwise.
- Show the equation, substitution and unit in your working.
Try the lesson
The start of the Equilibrium: (a) moment of force; moment = Fx lesson, made from checked teaching notes for this specification point.
Listen
The moment of a force is the turning effect of that force about a pivot. It is calculated as the product of the force and the perpendicular distance from the pivot to the line of action of the force: moment = Fx, where F is the force in newtons and x is the perpendicular distance in metres. The unit is the newton metre (N m). A larger force or a larger perpendicular distance gives a larger moment. If the force is not perpendicular to the lever, use the perpendicular component of the force or the perpendicular distance to the line of action. Moments are vector quantities in the sense that clockwise and anticlockwise moments oppose each other. A strong answer earns marks for these points. Moment of a force is the turning effect about a pivot. moment = Fx, where F is force and x is perpendicular distance from pivot to line of action. The unit of moment is the newton metre (N m). Increasing force or perpendicular distance increases the moment. Exam tip: Identify the pivot and draw the perpendicular distance before substituting values.
See it
Key words
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Moment of a force
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Equilibrium and opposing moments
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SI units and conversions
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See every definition
- Moment of a force
- The turning effect of that force about a pivot, calculated as the product of the force and the perpendicular distance from the pivot to the line of action of the force.
- Equilibrium and opposing moments
- Clockwise and anticlockwise moments oppose each other and are treated as opposing when applying equilibrium.
- SI units and conversions
- The unit of moment is the newton metre (N m), and distances must be converted from centimetres to metres before calculating.
Label the diagram
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