Law of Conservation of Mass

Law of conservation of mass states that for any given closed system, mass can neither be created nor be destroyed, over the time it can be conserved. It is also known as the principle of mass conservation.

The total mass of reactants should be equal to the total mass of the products for an isolated system with low energy thermodynamic process. It is believed that there are few assumptions from classical mechanics which defines mass conservation. Later the law was modified with the help of quantum mechanics and special relativity that energy and mass are one conserved quantity.

Formula for law of conservation of mass

Using classical mechanics law of conservation of mass can be formulated for isolated systems when energy scale is smaller than mc2


m: mass of an object in the system

C: speed of light

Conservation of mass can be expressed in the differential form using the continuity equation in fluid mechanics and continuum mechanics as

\(\frac{\partial \rho }{\partial t}+\bigtriangledown (\rho v)=0\)


ρ: density

t: time

v: velocity

⛛: divergence

Examples of law of conservation of mass

  • Combustion process: Burning of wood is a conservation of mass as the burning of wood involves Oxygen, Carbon dioxide, water vapor and ashes.
  • Chemical reactions: To get one molecule of H2O (water) with the molecular weight of 10, Hydrogen with molecular weight 2 is added with Oxygen whose molecular weight is 8, thereby conserving the mass.

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A simple pendulum of length l is suspended from the ceiling of a trolley which is moving, without friction, down an inclined plane of inclination .  The time period of the pendulum is given by T=2πlgeff where geff is given by