The relative velocity, denoted v → → B ∣ ∣ A (also v → → B A or v → → B rel ⁡ ⁡ A ), is the velocity vector of an object or observer B in the rest frame of another object or observer A. The relative speed v B ∣ ∣ A = ‖ ‖ v → → B ∣ ∣ A ‖ ‖ is the vector norm of the relative velocity.