 Though bending moments have been used to determine the stress states in arbitrary shaped structures, the physical interpretation of the computed stresses is problematic.However, physical interpretations of bending moments in beams and plates have a straightforward interpretation as the stress resultants in a cross-section of the structural element.For example, in a beam in the figure, the bending moment vector due to stresses in the cross-section "A" perpendicular to the "x"-axis is given by
Expanding this expression we have,
We define the bending moment components as 
The internal moments are computed about an origin that is at the neutral axis of the beam or plate and the integration is through the thickness (formula_22)

In the beam shown in the adjacent figure, the external forces are the applied force at point A (formula_23) and the reactions at the two support points O and B (formula_24 and formula_25).