Ok, I asked this before, but I was not happy with the anwser and I will ask it again. Lets say we have a rigid body. We section the body, and now there is an exposed plane with internal forces present, that become exposed to the section. If we look at a point on the exposed part, we want to know the stress at that point. So WHHYYYYYY do they use an elemental volume to do this???? You can do it much simpler by using a plane. Just find the stress along three mutually orthogonal planes that pass through the point. Then any plane will be some linear combination of these three plains. Why the heck are they using a elemental VOLUME to do the work of a PLANE. If you have an elemental volume, then the point your conrcerned with lies somewhere inside the volume. So that means the value of shear stress you calculate will be some SMALL distance away from the point you actually WANT. (Unless the point lies on one of the faces of the volume). In that case you would get the same anwser as i am stating with the use of planes, but you would get the most error using the other faces to find the shear in an orthogonal direction. This is really starting to bug me.