The law of conservation of mass can only be formulated in classical mechanics when the energy scales associated to an isolated system are much smaller than m c 2 , where m is the mass of a typical object in the system, measured in the frame of reference where the object is at rest, and c is the speed of light. The law can be formulated mathematically in the fields of fluid mechanics and continuum mechanics, where the conservation of mass is usually expressed using the continuity equation,