Conservation of momentum is a fundamental conservation law of physics. It states that the momentum of a system is constant if there are no external forces acting on the system.

In physics, conservation laws state that, in a closed system, the total quantity of momentum of the system remains constant; that is, its rate of change is zero. Mathematically, this is expressed as dX/dt = 0 for a scalar quantity X, and as div F = 0 for a vector quantity F. The most well-known conservation law is the law of conservation of energy, which states that the total energy of a closed system remains constant during a transformation. This is the first law of thermodynamics, and can also be stated as the total energy of a closed system is equal to the sum of its potential and kinetic energies. Another conservation law, for which no exception has yet...