Free-Body Diagram: Consider an Inclined Plane  
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The figure shows a prismatic bar with a load of magnitude P on the left end of the bar directed away from the end surface. An inclined section through the bar labeled b b is shown. The stresses on the inclined surface, labeled sigma, are directed to the right.

We will cut a free-body diagram along section b-b to investigate the stresses acting on an arbitrary internal plane in the bar.