Stress at a point
Suppose a rock that is in equilibrium is subjected to a force F as shown below. The force has no component in the x-direction. Consider a section through the rock; and assume that the stress is uniform throughout the section.
Now consider a small square in the section around point p, sketch the normal and shear stresses that are nonzero on this element below.
The orientation of your vector will be graded. The exact length of your vectors will not be grades make sure all of the vectors start at the black dot.