Moments are usually defined with respect to a fixed reference point; they deal with physical quantities as measured at some distance from that reference point. For example, the moment of force acting on an object, often called torque, is the product of the force and the distance from a reference point. In principle, any physical quantity can be multiplied by distance to produce a moment; commonly used quantities include forces, masses, and electric charge distributions. When a force acts on a particle, the motion that can occur is that of translation only i.e., the particle moves in a straight line. But forces acting on a rigid body may produce in it the following types of motion:
- Motion of Translation
- Motion of Rotation
- A combination of both
The tendency of a force to produce rotation about a fixed point is directly proportional to the product of the force P and the perpendicular distance p, of the line of action of the force from O. This product P × p is called the moment of the force P about O.The resultant force exists of two unequal and unlike parallel forces, with different line of actions acting on a rigid body. But if two equal and unlike parallel forces with different line of actions act on a rigid body, then the resultant force of the two cannot be found by combining these forces. Equivalently, no single force can replace two equal and opposite forces with different line of action. These kind of equal and opposite forces are said to form a couple.