Inertiales

Inertial frames of reference are coordinate systems in which an object either remains at rest or moves at a constant velocity unless acted upon by external forces. In Newtonian mechanics, this means Newton's first and second laws take their standard form without fictitious forces. According to Galileo's principle of relativity, the laws of physics are the same in all inertial frames, and no preferred frame exists.

Non-inertial frames are accelerating or rotating relative to an inertial frame and require fictitious forces such as the Coriolis and centrifugal forces to describe motion from that frame.

In practice, inertial concepts appear in measurement and navigation. Accelerometers measure proper acceleration, not coordinate acceleration, and are central to inertial navigation systems (INS) that track position by integrating acceleration data, often combined with external aiding sources like GPS. Gyroscopes measure rotation, enabling orientation tracking. Modern inertial sensors form the basis of consumer devices, aircraft avionics, spacecraft, and robotics.

Inertia also refers to the resistance of any object to changes in its state of motion, quantified by mass and, for extended bodies, by the moment of inertia relative to an axis of rotation.

Relativistic physics posits that all local experiments in any inertial frame yield the same results, and general relativity explains gravitation as geometry, with local inertial frames in curved spacetime. Practically, the term ‘inertial’ underpins classical mechanics as well as modern navigation, measurement, and physics research.