The stress tensor stores information about the stress on each of the faces of an infinitesimal volume of material. But I am confused as to why the stress components on opposite faces should be equal. The textbook I am reading (Fluid Mechanics by Kundu and Cohen) gives the following explanation:
On the opposite face EFGH the stress components have the same value as on ABCD, but their directions are reversed. This is because Figure 2.4 shows the stress at a point. The cube shown is supposed to be of “zero” size, so that the faces ABCD and EFGH are just opposite faces of a plane perpendicular to the x2-axis. That is why the stresses on the opposite faces are equal and opposite.
I am not sure why the cube being infinitesimal size implies that the stress components on opposite faces should be the same. Could someone please explain?