Relative velocity

The relative velocity of an object B with respect to an observer A, denoted ${\displaystyle \mathbf {v} _{B\mid A}}$ (also ${\displaystyle \mathbf {v} _{BA}}$ or ${\displaystyle \mathbf {v} _{B\operatorname {rel} A}}$), is the velocity vector of B measured in the rest frame of A. The relative speed is the vector norm of the relative velocity, ${\displaystyle v_{B\mid A}=\|\mathbf {v} _{B\mid A}\|}$.