The behavior of r θ θ ˙ ˙ 2 indicates that one must say that r θ θ ˙ ˙ 2 is not centrifugal force, but simply one of two terms in the acceleration. This view, that the acceleration is composed of two terms, is frame-independent: there is zero centrifugal force in any and every inertial frame. It also is coordinate-system independent, which means that one can use Cartesian, polar, or any other curvilinear system because they all produce zero.