text: Following the previous equation, The linéar momentum of the system is: formula_25 By definition the stress vector is formula_26, then formula_27 Using the Gauss's divergency theorem to convert a surface integral to a volume integral gives formula_28 formula_29 For an arbitrary volume the integrand itself must be zero, and we have the Cauchy's equation of motion formula_30 If a system is in equilibrium, the