Inertial frame of reference

A system in which observers move without the influence of any accelerating or decelerating force. If you can find an inertial frame of reference for a given situation, then there is always another one in which the observers do not move at all. The transition from one inertial frame of reference to another is done using [Lorentz transformations], or, at low energies where v << c, [Galilei transformations].