A moment of a force is the turning effect the forces causes about a fixed point, called the pivot. To calculate moment: Moment = Force × Perpendicular distance between Force and pivot. Moment is given in Nm. The moment of a force could cause a clockwise or an anticlockwise rotation. For an object to be in equilibrium, the total clockwise moment must be equal to the total anticlockwise moment about any pivot on the object. This is called the principle of moments. A lever is defined as a system consisting of a pivot, effort and a load. Gears are wheels with toothed edges that rotate on a shaft, with the teeth of one gear fitted into the teeth of the other. Since at the point of contact the two gears must be moving in the same direction, the two gears rotate in opposite directions.
A couple is a special case of moments. A couple is said to exist when two parallel forces of the same magnitude and opposite direction act on an object along different lines, producing a turning effect. The moment of a couple is called the torque and can be found by multiplying the magnitude of the force with the perpendicular distance between the forces.
The centre of mass of an object is the point at which all the mass of the object appears to act. For symmetrical objects, the centre of mass is the point of symmetry. The weight of the object can be represented as a downwards arrow starting from the centre of mass; this is called the line of action of the weight. For an object to be in equilibrium, the line of action of its weight must be inside its base; if not, the object will topple over. If an object is suspended from its centre of mass, it will balance; if not, it will come to rest with the centre of mass vertically below the point from which it is suspended.