Inclassical mechanics,linear momentumortranslational momentum(pl.momenta;SIunitkgm/s, or equivalently,Ns) is the product of themassandvelocityof an object. For example, a heavy truck moving rapidly has a large momentumit takes a large or prolonged force to get the truck up to this speed, and it takes a large or prolonged force to bring it to a stop afterwards. If the truck were lighter, or moving more slowly, then it would have less momentum.Like velocity, linear momentum is avectorquantity, possessing a direction as well as a magnitude:

wherepis the three-dimensional vector stating the object's momentum in the three directions of three-dimensional space,vis the three-dimensional velocity vector giving the object's rate of movement in each direction, andmis the object's mass.Linear momentum is also aconservedquantity, meaning that if aclosed systemis not affected by external forces, its total linear momentum cannot change.In classical mechanics,conservation of linear momentumis implied byNewton's laws. It also holds inspecial relativity(with a modified formula) and, with appropriate definitions, a (generalized) linear momentumconservation lawholds inelectrodynamics,quantum mechanics,quantum field theory, andgeneral relativity. It is ultimately an expression of one of the fundamental symmetries of space and time, that oftranslational symmetry.