Momentum

[1] In Newtonian mechanics, momentum (symbol: p) (pl.: momenta/momentums; more specifically linear momentum/translational momentum) is the product of an object's mass m and velocity v: p = mv. It is a vector quantity, possessing a magnitude and a direction. An object's momentum is notated as p from Latin pellere meaning "push, drive". Momentum's SI unit of measurement is the kilogram metre per second (kg⋅m/s), dimensionally equivalent to the newton-second. Newton's 2nd law says the change rate of a body's momentum equals the net force acting on it.

[1] Momentum depends on a reference frame, but in any inertial reference frame, it's a conserved quantity, i.e., if a closed system is unaffected by external forces, its total momentum won't change.

• inertial reference frame: viewpoint where objects are at rest/move at constant velocity unless force acts upon

  • reference frame: a viewpoint/coordinate system with an origin, directions, to measure an object's position/velocity/motion

  • inertial: not accelerating/at rest, moving at constant speed

Momentum is also conserved in special relativity (with a modified formula) and, in a modified form, in electrodynamics, quantum mechanics, quantum field theory, and general relativity. It is an expression of one of the fundamental symmetries of space and time: translational symmetry.