"Consider a prismatic member AB possessing a plane of symmetry and subjected to equal and opposite couples M and M0 acting in that plane . We observe that if a section is passed through the member AB at some arbitrary point C, the conditions of equilibrium of the portion AC of the member require that the internal forces in the section be equivalent to the couple M" Could anyone provide me the proof of this? It seems little weird to me. if we applie bending moment on a beam, it would likely that each end side would experience greater moment(away from centre of mass). How could every cross section have the same moment?