Moments of forces
Objective. Calculate signed moments about a chosen point.
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- 2. Worked example
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Learn: the key idea
A moment of a force measures its turning effect about a chosen point, calculated as the magnitude of the force multiplied by the perpendicular distance from the point to the line of action of the force, with units newton-metres (Nm). Moments are signed: conventionally anticlockwise is taken as positive and clockwise as negative, so moments about a point can be added algebraically. For a rigid body to be in rotational equilibrium, the sum of moments about any point must equal zero. For example, a force of 10 N acting 2 m from a pivot produces a moment of 20 Nm about that pivot.
Worked example
A force of 15 N acts perpendicular to a rod at a point 3 m from the pivot. Calculate the moment of this force about the pivot.
- 1Moment = force × perpendicular distance
- 2Force = 15 N, distance = 3 m
- 3Moment = 15 × 3
- 4Moment = 45 Nm
Answer: 45 Nm
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Start practisingCommon mistake. Students use the force multiplied by the distance along the rod rather than the perpendicular distance to the line of action. Always resolve to find the perpendicular distance, especially when forces act at an angle.
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