This is not a text book problem, instead it is how to understand a common diagram. The diagram occurs in the wiki entry for shear stress http://en.wikipedia.org/wiki/Shear_stress It shows a 2D square, anchored on the bottom side, and with positive (rightward) shear stress applied to the top side. The only stress shown explicitly is the top side shear stress. My question is, what other stresses are applied to the block that keep it in equilibrium, i.e. not moving and not rotating. The only other side where a stress can be applied is the bottom. So there must be a stress to counter the top stress to keep the square from moving rightward, and it will be negative and pointing leftward. But there must be other stresses to keep the block from rotating, as both these stresses apply counter-clockwise torques to the square. It seems to me that there must be an upward normal stress on the bottom right side and a downward normal stress on the left side of the bottom, so that the stress is not constant on the bottom. This is all new to me and I'm not confident of my understanding of the matter.