This means that the internal forces acting on the cross-section of the beam can be represented by one resultant force, called a shear force, that is the resultant of the internal shear forces, and by one resultant moment, called a bending moment, that is the resultant of the internal normal forces. These forces cancel each other out so they don’t produce a net force perpendicular to the beam cross-section, but they do produce a moment. Normal internal forces are either tensile or compressive The bottom of the beam will get longer, and so the normal forces acting at the bottom of the beam will be tensile. If the beam is sagging the top of the beam will get shorter, and so the normal forces acting at the top will be compressive. The normal stresses will be tensile on one side of the cross-section, and compressive on the other. No matter where the imaginary cut is made along the length of the beam, the effect of the internal forces will always balance the effect of the external forces. The internal forces develop in such a way as to maintain equilibrium. Shear (left) and normal (right) internal forces
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