¹⁵Derivation of this can be found in standard text books in continuum mechanics like [1, 2]. For understanding these derivation a lot more concepts needs to be grasped which is beyond the scope of this course. In fact, the following derivation here is also not easy to follow; but made as simple as possible. To proceed further in this course, it suffices to understand that there is a formal procedure to obtain constitutive relations for materials with constraints.