Non-inertial reference frame
In theoretical physics, a common coordinate system or frame of reference, that refers to a non-inertial state of motion can be referred to as a noninertial frame (as opposed to an inertial frame).
Simple noninertial frames
The two main two categories of noninertial frame are:
- Accelerated frames, where the reference coordinate system is usually appears to have uniform acceleration with respect to any inertial frame, and
- Rotating frames, where the reference coordinate system rotates about an axis compared to an inertial coordinate system.
Both of these case can be easily distinguished from inertial frames in a straightforward way, by the presence of geeforces.
Other forms of frame
When explicit gravitational fields or spacetime curvature is involved, these definitions can become less clear-cut.
An object free-falling in a gravitational field might be said to be accelerating (in the sense that it is changing velocity when its motion is plotted in a conventional inertial frame), but is also "not accelerated", since it feels no geeforces. The object could legitimately be said to be following a curved trajectory because of the action of a gravitational force, or to be following an inertial path through curved spacetime (a geodesic) with no forces acting upon it. Since it is unlikely that there will be similar distant objects with the same motion, the idea of a conventional global inertial frame is often not useful when dealing with these problems.
It may sometimes be useful to refer to a freefall frame for making comparisons in a vertical cylindrical or inverted conical section of space around the object, extending form the centre of the gravity-source to the outside universe, or we might choose to describe physics in the immediate locale of the falling object with a local inertial frame.
Alternatively, if a series of objects are suspended at different heights in the gravitational field, then although they well be stationary in a global inertial frame, inertial frame rules will not apply: the objects are not inertial, and physical attempts to construct a calibrated coordinate system between them will be affected by the presence of the gravitational field. These sorts of stationary accelerated coordinate systems are sometimes used to study the expected exterior properties of black holes.