In discussion of a particle moving in a circular orbit, in an inertial frame of reference, one can identify the centripetal and tangential forces. Some fictitious forces, commonly called the centrifugal and Euler force, underlines this switch in vocabulary, and it is a change of observational frame of reference from the inertial frame, where centripetal and tangential forces make sense, to a rotating frame of reference where the particle appears motionless and fictitious centrifugal and Euler forces have to be brought into play.