Relative velocity
The relative velocity of an object B with respect to an observer A, denoted v B ∣ A {\displaystyle \mathbf {v} _{B\mid A}} (also v B A {\displaystyle \mathbf {v} _{BA}} or v B rel A {\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, v B ∣ A = ‖ v B ∣ A ‖ {\displaystyle v_{B\mid A}=\|\mathbf {v} _{B\mid A}\|} . == Classical Mechanics == === In One Dimension (non-relativistic) === We begin with relative motion in the classical, (or non-relativistic, or the Newtonian approximation) that all speeds are much less than the speed of light.