Introducing a unit vector n = v / v = β / β in the direction of relative motion, the relative velocity is v = v n with magnitude v and direction n, and vector projection and rejection give respectively r ∥ ∥ = ( r ⋅ ⋅ n ) n , r ⊥ ⊥ = r − − ( r ⋅ ⋅ n ) n