Moments

This lesson covers moments (turning effects of forces) at A-Level. A moment quantifies the tendency of a force to cause rotation about a point or axis. Understanding moments is essential for solving problems involving rigid bodies in equilibrium.

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Definition of a Moment

The moment of a force about a point is:

$\text{Moment} = F \times d$Moment=F×d

where:

• $F$F is the magnitude of the force (in newtons)
• $d$d is the perpendicular distance from the line of action of the force to the pivot point (in metres)

The unit of a moment is newton-metres (Nm).

Direction: Moments are either clockwise or anticlockwise about the pivot.

Exam Tip: The distance $d$d must be the perpendicular distance from the pivot to the line of action of the force. If the force is not perpendicular to the line from the pivot to the point of application, you must resolve the force or find the correct perpendicular distance.

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The Principle of Moments

For a body in rotational equilibrium (not rotating or rotating at constant angular velocity):

$\text{Sum of clockwise moments} = \text{Sum of anticlockwise moments}$Sum of clockwise moments=Sum of anticlockwise moments