$2\mathrm{.}51*$ The rectangular cross section $A$ is subjected to a normal stress given by the

equation $=ay,$ where $a$ is a constant. At $y=0\mathrm{.}8m,=20$MPa. Determine

$($a$)$ the average normal stress on A and $($b$)$ the magnitude of the moment about

the $z$ axis exerted by the normal stress.

Problem $2\mathrm{.}51$