# Velocity-Stress formulation of Elastodynamics/Wave Equation for beginner

I'm used to displacement forumlation of elastic wave equation: $$\nabla \cdot \sigma (u) + F = \rho \ddot{ u }$$ where $u$ is the primary variable. Recenty I started experimenting with DG and in almost every paper the stress-velocity formulation is used as a "conservative or divergence form". What's special about this formulation? I can't figure out why they use Inerior Penalty with displacement equation and other fluxes like Lax-Friedrich's with a system of $v, \sigma$. I would be thankful for any kind of paper, book or resource.