Free-Body Diagram: Consider Vertical Plane  
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The figure shows a prismatic bar subjected to loads on either end. The loads have magnitude P and are directed away from the end surfaces of the bar. A section labeled a a parallel to the cross section of the bar is shown.
 

Consider a bar subjected to an axial force P. Force P is applied at the centroid of the bar. The cross-sectional area of the bar is A.

To investigate the stresses that are acting internally in the material, we will cut through the bar at a-a.