Understanding Internal Moments in Mechanics of Materials

[Cyrus]

In mechanics of materials we often look at elemental cubes or planar squares, and we write the normal stress and shear stress acting on the faces. Why are the internal moments acting on each face omited? When we cut a frame-machine, we include a shear, normal and moment, why not for the elemental cube/plane?

I figured it out, someone please delete this pointless dribble.

 

[sruthisupriya]

will it be because the internal moments cancel off each other?

 

[Cyrus]

No, the internal moment is actually a resultant of a distributed contiuum of forces acting along the face where the cut is made. So you can effectively get rid of the internal moment by drawing a distribution of the contiuum, as opposed to a resultant normal, shear and moment. Its just looking at the normal, shear and moment in a different light.