The normal stress acting on section $\mathrm{a}-\mathrm{a}$ can be calculated from $\mathrm{\sigma }=\mathrm{P}/\mathrm{A}$. Section $\mathrm{a}-\mathrm{a}$ is unique, however, because it is the only surface that is perpendicular to the direction of the force $\mathrm{P}$. A more general case would be to consider a section cut through the bar at an arbitrarily chosen angle.