Equations of motion

Now the linear momentum principle may be applied to an arbitrary finite body. Using the expression for T_j above and the divergence theorem of multivariable calculus, which states that integrals over the area of a closed surface S, with integrand n_i f (x), may be rewritten as integrals over the volume V enclosed by S, with integrand ∂f (x)/∂x_i; when f (x) is a differentiable function, one may derive thatat least when the σ_ij are continuous and differentiable, which is the typical case. These are the equations of motion for a continuum. Once the above consequences of the linear momentum principle are accepted, the only further result that can be derived from the angular momentum principle is that σ_ij = σ_ji (i, j = 1, 2, 3). Thus, the stress tensor is symmetric.