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The law of conservation of mass can only be formulated in classical mechanics, in which the energy scales associated with an isolated system are much smaller than , where is the mass of a typical object in the system, measured in the frame of reference where the object is at rest, and is the speed of light.
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time. Exact conservation laws include conservation of mass-energy , conservation of linear momentum , conservation of angular momentum , and conservation of electric charge .
In physics, a mass balance, also called a material balance, is an application of conservation of mass [1] to the analysis of physical systems.By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique.
Conservation of mass, although not exact, is extremely important in sciences that does not deal with relativity (i.e. chemistry) and also physics from a historical perspective. The section on "Mass conservation in the theory of special relativity" lacks context and overshadows the importance of mass conservation.
The compressible Euler equations consist of equations for conservation of mass, balance of momentum, and balance of energy, together with a suitable constitutive equation for the specific energy density of the fluid. Historically, only the equations of conservation of mass and balance of momentum were derived by Euler.
The conservation of mass is expressed locally by the fact that the flow of mass density satisfies the continuity equation: + =, where is the mass flux vector. The formulation of energy conservation is generally not in the form of a continuity equation because it includes contributions both from the macroscopic mechanical energy of the fluid flow and of the microscopic internal energy.
The Nernst–Planck equation is a conservation of mass equation used to describe the motion of a charged chemical species in a fluid medium. It extends Fick's law of diffusion for the case where the diffusing particles are also moved with respect to the fluid by electrostatic forces.
Rockets, which lose significant amounts of mass as fuel during flight, are an example of a variable-mass system. In mechanics, a variable-mass system is a collection of matter whose mass varies with time. It can be confusing to try to apply Newton's second law of motion directly to such a system.