Final answer:
Shear stress τxy is equal to shear stress τyx, due to the principle of transposed shear stresses, which is a reflection of the equilibrium of forces in a material.
Step-by-step explanation:
In the context of material science and engineering mechanics, shear stresses on an element of material are mutually related. According to the principle of transposed shear stresses, shear stress τxy is equal to shear stress τyx. This relationship indicates that the sideways stress on the face of the material with normal in the x-direction due to forces in the y-direction is the same as the sideways stress on the face with normal in the y-direction due to forces in the x-direction. This is a consequence of the equilibrium of forces in a continuum material, assuming there are no body forces acting and the material is isotropic.