| The equilibrium equations describe how the beam
carries external pressure loads with its internal stresses. Rather
than deal with these stresses themselves, we choose to work with the
resultants since they are functions of x only (and not of
y).
To enforce equilibrium, consider the balance of forces and
moments acting on a small section of beam,
Equilbrium in the y-direction gives the equation for the
shear resultant V,
Moment equilibrium about a point on the right side of the beam
gives the equation for the moment resultant M,
Note that the pressure load p does not contribute to the
moment equilibrium equation. | 
