Let us model a rigid bar as a linear monatomic chain of atoms, as in Sect. 6.1.3 with the same notations. We further assume that the equilibrium interatomic separation is a and that its cross section is a₂. Its Young’s modulus E_Y is defined as the ratio of the stress applied in one direction divided by the relative elongation in this same direction. The stress is the ratio of the interatomic force (F_n,n^-1) divided by the cross-sectional area (a₂) on which this force is applied. The relative elongation is the interatomic displacement divided by the equilibrium separation. The Young’s modulus has the dimension of a pressure and is expressed in Pa (Pascal). The solid density M_V is the ratio of the mass of the solid to its volume. Here, we assume that the mass of an atom is M and that there is only one atom in a volume of a₃.