The resultant force produced by the bending stress on each numbered portion of the beam cross section has been determined. Next, we must determine the magnitude of the internal moment M produced by the bending stresses.
To do this, we must determine the location of each resultant force relative to the neutral axis (i.e., the z axis). The internal moment produced by each resultant force is the product of the force magnitude and the distance between the neutral axis and the resultant force.