Resultant of Normal Stresses on Vertical Plane  
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The figure shows a section of a prismatic bar. Equal magnitude loads on either end have magnitude P and are directed away from the surface of the bar. The stresses on a section labeled a a are directed rightwards.
 

The bar is in equilibrium; therefore, the resultant of the internal force distribution must equal the force P applied to the bar.

Note that no shear stress τ acts on section a-a since the exposed surface is perpendicular to the direction of the force P.