The term in parentheses is the original expression of r in Cartesian coordinates. Consequently, a = − − ω ω 2 r . negative shows that the acceleration is pointed towards the center of the circle (opposite the radius), hence it is called "centripetal" (i.e. "center-seeking"). While objects naturally follow a straight path (due to inertia), this centripetal acceleration describes the circular motion path caused by a centripetal force.