law of conservation of mass
A law that states that mass cannot be created or destroyed; it is merely rearranged.
Examples of law of conservation of mass in the following topics:

The Law of Conservation of Mass
 The law of conservation of mass states that mass in an isolated system is neither created nor destroyed.
 However, Antoine Lavoisier described the law of conservation of mass (or the principle of mass/matter conservation) as a fundamental principle of physics in 1789.
 This law was later amended by Einstein in the law of conservation of massenergy, which describes the fact that the total mass and energy in a system remain constant.
 However, the law of conservation of mass remains a useful concept in chemistry, since the energy produced or consumed in a typical chemical reaction accounts for a minute amount of mass.
 A portrait of Antoine Lavoisier, the scientist credited with the discovery of the law of conservation of mass.

The Law of Multiple Proportions
 The law of multiple proportions states that elements combine in small whole number ratios to form compounds.
 The law of multiple proportions, also known as Dalton's law, was proposed by the English chemist and meteorologist John Dalton in his 1804 work, A New System of Chemical Philosophy.
 The law, which was based on Dalton's observations of the reactions of atmospheric gases, states that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small whole number ratios.
 Dalton's law of multiple proportions is part of the basis for modern atomic theory, along with Joseph Proust's law of definite composition (which states that compounds are formed by defined mass ratios of reacting elements) and the law of conservation of mass that was proposed by Antoine Lavoisier.
 These laws paved the way for our current understanding of atomic structure and composition, including concepts like molecular or chemical formulas.

Balancing Nuclear Equations
 To balance a nuclear equation, the mass number and atomic numbers of all particles on either side of the arrow must be equal.
 Nuclear reactions may be shown in a form similar to chemical equations, for which invariant mass, which is the mass not considering the mass defect, must balance for each side of the equation.
 The transformations of particles must follow certain conservation laws, such as conservation of charge and baryon number, which is the total atomic mass number.
 The result is an atomic mass difference of 4 and an atomic number difference of 2.
 In order to solve this equation, we simply add the mass numbers, 214 for polonium, plus 8 (two times four) for helium (two alpha particles), plus zero for the electrons, to give a mass number of 222.

Relativistic Energy and Mass
 In special relativity, an object that has a mass cannot travel at the speed of light.
 Relativistic corrections for energy and mass need to be made because of the fact that the speed of light in a vacuum is constant in all reference frames.
 The conservation of mass and energy are wellaccepted laws of physics.
 In order for these laws to hold in all reference frames, special relativity must be applied.
 While Newton's second law remains valid in the form the derived form is not valid because in is generally not a constant.

Relativistic Momentum
 Conservation laws in physics, such as the law of conservation of momentum, must be invariant.
 That is, the property that needs to be conserved should remain unchanged regardless of changes in the conditions of measurement.
 This means that the conservation law needs to hold in any frame of reference.
 Newton's second law [with mass fixed in the expression for momentum (p=m*v)], is not invariant under a Lorentz transformation.
 However, it can be made invariant by making the inertial mass m of an object a function of velocity:

Conservation of Nucleon Number and Other Laws
 In physics and chemistry there are many conservation laws—among them, the Law of Conservation of Nucleon Number, which states that the total number of nucleons (nuclear particles, specifically protons and neutrons) cannot change by any nuclear reaction.
 Alpha decay is the only type of radioactive decay that results in an appreciable change in an atom's atomic mass.
 Chain reactions of nuclear fission release a tremendous amount of energy, but follow the Law of Conservation of Nucleon Number.
 In each step, the total atomic mass of all species is a constant value of 236.
 Finally, nuclear fusion follows the Law of Conservation of Nucleon Number.

Linear Momentum
 Linear momentum is the product of the mass and velocity of an object, it is conserved in elastic and inelastic collisions.
 In classical mechanics, linear momentum, or simply momentum (SI unit kg m/s, or equivalently N s), is the product of the mass and velocity of an object.
 "Newton's cradle" shown in is an example of conservation of momentum.
 As we will discuss in the next concept (on Momentum, Force, and Newton's Second Law), in classical mechanics, conservation of linear momentum is implied by Newton's laws.
 Total momentum of the system (or Cradle) is conserved.

Momentum, Force, and Newton's Second Law
 This fact, known as the law of conservation of momentum, is implied by Newton's laws of motion.
 If the mass of the system is constant, then Δ(mv)=mΔv.
 So for constant mass, Newton's second law of motion becomes
 Newton's second law of motion stated in terms of momentum is more generally applicable because it can be applied to systems where the mass is changing, such as rockets, as well as to systems of constant mass.
 Therefore, the total momentum of the balls is conserved.

Properties of Electric Charges
 Electric charge, like mass and volume, is a physical property of matter.
 Like mass, electric charge in a closed system is conserved.
 This is known as Coulomb's Law.
 The formula for gravitational force has exactly the same form as Coulomb's Law, but relates the product of two masses (rather than the charges) and uses a different constant.
 Describe properties of electric charge, such as its relativistic invariance and its conservation in closed systems

Conservation of Energy and Momentum
 While inelastic collisions may not conserve total kinetic energy, they do conserve total momentum .
 where $v_1$ is the initial velocity of the first mass, $v{}'_1$ is the final velocity of the first mass, $v_2$ is the initial velocity of the second mass, and $\theta {}'_1$ is the angle between the velocity vector of the first mass and the xaxis.
 Since there are no net forces at work (frictionless surface and negligible air resistance), there must be conservation of total momentum for the two masses.
 By applying conservation of momentum in the ydirection we find:
 GCSE physics  how to calculate momentum and use the conservation of momentum law.