Problem $3$:

$4\mathrm{.}24.[4\mathrm{.}8]$ A rod held between two walls $($Fig$.$ P$\mathrm{.}4\mathrm{.}24)$ is heated from $30C$ to $120C.$ What is the stress in the rod if $=18\frac{{10}^{-6}}{}C$ and the walls move apart a distance of $\mathrm{.}3$ mm $?$ Take $E=1\mathrm{.}4{10}^{11}Pa.$

Figure P$\mathrm{.}4\mathrm{.}24.$