2Moments and Couples
2.1 Types of Moments
Moments can be classified as internal moments, external moments, and reaction moments.
External Moments
An external moment can be defined as a moment acting on the surface of the body and transmitted to the body by contact, i.e. by a force acting external to the body. An external moment applied at a point is called a concentrated moment, and applied over a surface it is called a distributed moment.
Internal Moments
Internal forces acting on a body are independent of whether or not external forces are applied on the body. When an external force is applied to a body, internal forces occur inside the object causing stresses. When an external moment is applied to a body, the result will be either torsion or bending. The process introduces internal moments in the body. Internal moments about an axis are either torsional moments, i.e. torque, or respectively bending moments, depending on the axis direction with respect to the imaginary cut surface, either normal or respectively tangent.
Reaction Moments
Moments can be developed at the supports in order to resist movement (rotation) due to the applied external forces or moments.
2.2 Moment of a Force About a Point
Consider a force 
and a given point 
located on the plane (Figure 2.1) defined ...
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