If we consider a differential element dA' of the subelement, we can express the normal force acting in the x direction due the bending stress s as a differential force dF = σ dA'.
The resultant of these differential forces in the x direction on a face of the beam element is found by integrating the normal stress over the area A':
where σ is the flexural stress at a distance y from the neutral surface. The flexural stress is given by the expression
