The turning effect of a force is called the moment of a force or torque. The moment of a force is defined as the turning effect of the force about a pivot, and is calculated by the product of the force F, and the perpendicular distance d, from the line of action of the force to the pivot. The SI unit for the moment of a force is the newton metre (Nm). The moment of a force can be clockwise of anticlockwise.
Moment of a force (Nm) = force (F) × perpendicular distance from the line of action to the pivot (d)
The moment of a force can be increased by:
i. increasing the force applied; ii. increasing the perpendicular distance from the line of action to the pivot.
Everyday examples of moments are:
opening a bottle with an opener; opening a door; steering wheel.
Levers
A crowbar is a lever that can be used to raise one end of a heavy object. The weight of the object is called the load, the force applied to the crowbar is called the effort and the point about which the crowbar turns is called the pivot (or fulcrum).
The principle of Moments
This principle is about objects balancing when the moments of all the forces acting are balanced. At balance the sum of the clockwise moments about the pivot is equal to the sum of the anticlockwise moments. The principle of moments states:
For an object in equilibrium, the sum of the clockwise moments about any point (acting as a pivot) equals the sum of the anticlockwise moments about the same point.
The principle of moments is related to the centre of gravity and the stability of an object