Resultant of Normal Stresses on Inclined Plane  
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The figure shows a prismatic bar with loads of magnitude P on either end directed away from the end surfaces. An inclined section through the bar labeled b b is shown. The stresses on the inclined surface, labeled sigma, are directed to the right.

Because the stresses are the same throughout the entire bar, the stresses on the inclined surface must be uniformly distributed.

The bar is in equilibrium. Consequently, the resultant of the uniformly distributed stress must equal P, even though the stress act on the inclined surface b-b.