1. The problem statement, all variables and given/known data Why cant i use the equation to determine the normal stress and shear stress ? Is my concept wrong ? I use the method in the link to do this question. The question said the stress on a plane making angle 30 degree anticlockwise from plane AD . 2. Relevant equations 3. The attempt at a solution When it's anticlockwise , the angle is positive http://www.dummies.com/education/science/using-mohrs-circle-to-find-principal-stresses-and-angles/ Mod note: Image of work has been deleted. OP will be including work in a subsequent post.