I am a civil engineer and have been in the field for 10 years so I have forgotten some basic theories of Maths.
For me, what I remember is // axis theorem is able to make second moment of area of a section with reference with respect to any axis. In the basic bending theory of beam, the longitudinal bending stress is equal to M y / I where M is bending moment, y is the perpendicular distance to the neutral axis and I is the second moment of inertia.
(Pls see this webpage for details: Bending) - Wikipedia, the free encyclopedia. As y is the distance measuring form the neutral axis, the stress is not zero at both extremities. I was wondering whether I can shift the NA downward to the bottom of a section so that the bending stress is bottom is “zero”