This article describes a particle in planar motion when observed from non-inertial reference frames.

When in a non-inertial reference frame, fictitious forces, such as centrifugal pseudoforce may be appropriate.

All non-inertial reference frames exhibit fictitious forces.

The force F does not arise from any physical interaction but rather from the acceleration a of the non-inertial reference frame itself.

However, in the rider's non-inertial reference frame attached to the accelerating car, there is a backward fictitious force.

In classical physics, fictitious forces are used to describe acceleration in non-inertial reference frames.

Inertial and non-inertial reference frames can be distinguished by the absence or presence of fictitious forces, as explained shortly.

Since non-inertial reference frames do not abide by the special principle of relativity, such situations are not self-contradictory.

This example illustrates how fictitious forces arise from switching from an inertial to a non-inertial reference frame.

(Thus, within a certain non-inertial reference frame, which is rotating at , the spiral arms appear to be at rest).

Less often it may refer to an arbitrary non-inertial reference frame; in particular, a Rindler frame is sometimes called an "accelerating observer".

A non-inertial reference frame is a frame of reference that is undergoing acceleration with respect to an inertial frame.

Switch to a non-inertial reference frame (e.g. when a satellite's orbit is described in a reference frame associated with the precessing equator of the planet).

Because a rotating frame is an example of a non-inertial reference frame, Newton's laws of motion do not accurately describe the dynamics within the rotating frame.

For solving problems of mechanics in non-inertial reference frames, the advice given in textbooks is to treat the fictitious forces like real forces and to pretend you are in an inertial frame.

However, in the non-inertial reference frame of Earth, treating a spot on the Earth as a fixed point, the stars are observed to move in the sky, circling once about the Earth per day.

The centrifugal force is sometimes called a fictitious force or pseudo force, to underscore the fact that such a force only appears when calculations or measurements are conducted in non-inertial reference frames.

This can occur only in a reference frame which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction (that is, a non-inertial reference frame).

Bodies in non-inertial reference frames are subject to so-called fictitious forces (pseudo-forces); that is, forces that result from the acceleration of the reference frame itself and not from any physical force acting on the body.

Measurements with respect to non-inertial reference frames can always be transformed to an inertial frame, incorporating directly the acceleration of the non-inertial frame as that acceleration is seen from the inertial frame.

Since the stars are light years away, this observation means that, in the non-inertial reference frame of the Earth, anybody who looks at the stars is seeing objects which appear, to them, to be moving faster than the speed of light.

By contrast, in a non-inertial reference frame the laws of physics vary depending on the acceleration of that frame with respect to an inertial frame, and the usual physical forces must be supplemented by fictitious forces.

For the following formalism, the rotating frame of reference is regarded as a special case of a non-inertial reference frame that is rotating relative to an inertial reference frame denoted the stationary frame.

Physics in non-inertial reference frames was historically treated by a coordinate transformation, first, to an inertial reference frame, performing the necessary calculations therein, and using another to return to the non-inertial reference frame.

Observers inside a closed box that is moving with a constant velocity cannot detect their own motion; however, observers within an accelerating reference frame can detect that they are in a non-inertial reference frame from the fictitious forces that arise.