If the force acting on the block is at an angle, we can separate the force into two components, one perpendicular to the surface (Ft), the other parallel to it (Fs). The component perpendicular to the surface (or normal to the surface) is creating a tensile stress and thus it’s magnitude is Ft/A
The other component of the stress, which is acting parallel to the surface is a shear stress, τ, and it’s magnitude is given by:
