Conservation of momentum

The principle that a system’s total momentum remains constant when no net external force acts on it. Momentum can pass between parts of the system while the total stays unchanged.

Momentum: A vector quantity equal to an object's mass multiplied by its velocity. The conserved total is the vector sum of the momenta of all parts of the system.
Elastic collision: A collision in which total kinetic energy and total momentum are conserved. Momentum conservation combines with kinetic-energy conservation to determine post-collision velocities.
Newton's laws of motion: Three classical laws relating forces, motion, and the response of bodies. The momentum principle is a system-level consequence of Newtonian dynamics.
Conservation of energy: The principle that total energy remains constant in an isolated system. Momentum is conserved under zero net external force; kinetic energy need not be.
Newton's third law: The law that interacting bodies exert equal and opposite forces on each other. Equal and opposite internal forces produce cancelling changes in system momentum.
Inelastic collision: A collision in which kinetic energy is not conserved, though total momentum is conserved in an isolated system. It shows that momentum can remain constant even when kinetic energy becomes heat, sound, or deformation.
External force: A force exerted on a system by something outside the chosen system boundary. The condition for constant total momentum is zero net external force.
Conservation of angular momentum: The principle that total angular momentum remains constant when net external torque is zero. It has a parallel symmetry and conservation rule, but concerns rotational motion and torque.
Impulse: The change in momentum produced by a force acting over a time interval. External impulse changes total momentum; zero net external impulse preserves it.
Recoil: The backward motion of an object when it expels matter or interacts with another object. The expelled matter's momentum is balanced by the object's change in momentum.

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