Now introduce the unit vectors e A and e B from the fulcrum to the point A and B, so r A − − r P = a e A , r B − − r P = b e B . This notation allows us to define the velocity of the points A and B as v A = θ θ ˙ ˙ a e A ⊥ ⊥ , v B = θ θ ˙ ˙ b e B ⊥ ⊥ , where e A and e B are unit vectors perpendicular to e A and e B, respectively.