Forced Changes Only: A New Take on the Law of Inertia

Newton’s First Law of Motion is typically understood to govern only the motion of force-free bodies. This paper argues on textual and conceptual grounds that the law is in fact a stronger, more general principle. The First Law limits the extent to which any body can change its state of motion—even if that body is subject to impressed forces. The misunderstanding can be traced back to an error in the first English translation of Newton’s Principia, which was published a few years after Newton’s death.