Answer:
Stress is a tensor quantity and tensor quantities require 2 sub scripts for their complete definition. 1 subscript defines force and the other area.
Explanation:
Stress at a point is defined as follows
[tex]\sigma =\lim_{\Delta a\rightarrow 0}\frac{\overrightarrow{\Delta F}}{\Delta A}[/tex]
In the expression above in addition to the direction of force F we also need to define orientation of the area that we choose in defining the limit of the quantity on right hand side of the expression above hence we get a dobule subscript notation in stress at a point. 1 subscript defines force and the other area.
