Jonstruct an appropriate Cartesian coordinate system $(x-y-z)$ and then

express each of following stress state in the form of a $33$ matrix.

$($i$)$ A point in a rock $10$ m underwater;

$($ii$)$ At the middle point of a pillar with height of $5$ m $,$ diameter of $0\mathrm{.}5$ m and

density of $2500k\frac{g}{m^3}$;

$($iii$)$ A point in the membrane $($thickness $=0\mathrm{.}1mm)$ of a balloon with internal

pressure of $0\mathrm{.}2$ MPa and diameter of $20$ cm $.$ Discuss the difference of the

stress if the point is located near the outer surface or the inner surface.