Moments and Torque

So far we have treated objects as particles — points with mass but no size. In reality, forces can act at different points on an object, and this matters. A force applied at the edge of a door swings it open easily; the same force applied near the hinge barely moves it. The turning effect of a force depends not just on its magnitude, but also on where and how it is applied. This is the concept of the moment of a force.

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Moment of a Force

The moment of a force about a point (or pivot) is defined as:

Moment = Force x Perpendicular distance from the line of action of the force to the pivot

Moment = F d

where F is the force in newtons and d is the perpendicular distance in metres. The unit of moment is the newton-metre (N m).

The line of action of a force is an imaginary line extending along the direction of the force. The perpendicular distance is measured at right angles from this line to the pivot.