Inertial frame of reference

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In classical physics and special relativity, an inertial frame of reference (also called inertial space, or Galilean reference frame) is a frame of reference with constant velocity. In such a frame of reference an object with zero net force acting on it is perceived to move with a constant velocity or, equivalently, it is a frame of reference in which Newton's first law of motion holds.^[1] All inertial frames are in a state of constant, rectilinear motion (straight line motion) with respect to one another; in other words, an accelerometer moving with any of them would detect zero acceleration.

Measurements of objects in motion (but not subject to forces) in one inertial frame can be converted to measurements in another by a simple transformation - the Galilean transformation in Newtonian physics or by using the Lorentz transformation (combined with a translation) in special relativity^[2] when the relative speed of the frames is more than about 10% of the speed of light.

For example, the centrifugal effect causes a fictitious force, which appears to pull objects away from the axis of rotation when the observer is rotating.

As another example, in classical mechanics, a ball dropped towards the ground does not seem move exactly straight down because the Earth's surface is not inertial (This is caused by the Earth's rotation). As a consequence, the Coriolis effect—an apparent force— must be taken into account to predict the respective small horizontal motion.

In a non-inertial reference frame, viewed from a classical physics and special relativity perspective, the interactions between the fundamental constituents of the observable universe (the physics of a system) vary depending on the acceleration of that frame with respect to an inertial frame. Viewed from this perspective and due to the phenomenon of inertia, the 'usual' physical forces between two bodies have to be supplemented by apparently source-less inertial forces.^[3][4] Viewed from a general relativity theory perspective, appearing inertial forces (the supplementary external causes) are attributed to geodesic motion in spacetime.