The

relativistic linear momentum of a body of mass

*m* and speed

*v* is

**Note: Bold letters are designated as vectors.*

**p** = m**v** /
√[ 1 - (v/c)^{2} ] = γm**v**

where γ = 1 / √[ 1 - (v/c)^{2} ] and γ > 1. Some prefer to associate the γ with the mass and introduce a relativistic mass *m*_{R} = γ*m*.. That allows you to write the momentum as p = m_{R}v, but *m*_{R} is speed dependent. Usually one mass *m* is used, which is independent of speed, just like the two other fundamental properties of particles of mattter, charge and spin.