Beyond this problem, the real impressed net force is zero. (There is no real impressed force in straight-line motion at constant speed). If one were to adopt polar coordinates, and wish to say that r θ θ ˙ ˙ 2 is "centrifugal force", and reinterpret r ¨ ¨ as "acceleration", the oddity results in frame S' that straight-line motion at constant speed requires a net force in polar coordinates, but not in Cartesian coordinates. Moreover, this perplexity applies in frame S ', but not in frame S.