To calculate the the stress on the cube, we use the formula
[tex]Stress =\frac{F}{A}[/tex]
Here, F is the normal force on the cube and A is surface area of the cube.
We know that every cube has six faces, therefore surface area of the cube,
[tex]A= 6\times l^2[/tex]
But according to question, the force is subjected to normal on the cube, therefore we take only one face of cube, then the surface area of the cube,
[tex]A = (l)^2[/tex]
As given, [tex]l= 3 m \ so,A=(3)^2=9 m^2[/tex]
Given [tex]F=36 N[/tex]
Thus, [tex]stress= \frac{36}{9} = 4\ N/m^2[/tex]
