Absolute and relative Frenet-Serret frames and Fermi-Walker transport

Relative Frenet-Serret 3-frames are defined along a test particle worldline with respect to a family of observers and a corresponding comoving relative Frenet-Serret 3-frame is introduced in the local rest space of the test particle itself The latter frame provides the tools to geometrically define a generalized centrifugal force, tied to the rotation of the relative velocity of the test particle with respect to the observers relative to gyro-fixed awes in the test particle local rest space. For circular orbits in a stationary axisymmetric spacetime, all of these relative frames for any circular observers are closely related to the spacetime Frenet-Serret frame, but it is the comoving frame, which reveals the geometric interpretation of the minimal intrinsic rotation observers, which are complementary to the extremely accelerated observers to which they reduce in the equatorial plane of rotating black hole spacetimes.