Equilibrium Equation  
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The figure shows an aluminum bar of dimensions 30 millimeters by 24 millimeters sandwiched between copper bars of dimensions 30 millimeters by 12 millimeters above and below it. The bars are held in place by pins on either end.

Draw a free-body diagram (FBD) that cuts through the bars and exposes the internal axial forces.

The equilibrium equation can be written as: