Momentum

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 momentum—it 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:

[\mathbf{p} = m \mathbf{v}.]

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; but 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.