Inertial frame of reference

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In Newtonian physics and special relativity, an inertial frame of reference (or Galilean reference frame) is a frame of reference in which Newton's first law of motion applies: an object moves at a constant velocity unless acted on by an external force. All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not accelerating (in the sense of proper acceleration that would be detected by an accelerometer). Measurements in one inertial frame can be converted to measurements in another by a simple transformation (the Galilean transformation in Newtonian physics and the Lorentz transformation in special relativity). In general relativity, an inertial reference frame is only an approximation that applies in a region that is small enough for the curvature of space to be negligible. [1][2]

Physical laws take the same form in all inertial frames.[3] In a non-inertial reference frame the laws of physics depend upon the particular frame of reference, and the usual physical forces must be supplemented by fictitious forces. [4] [5] For example, a ball dropped towards the ground does not go exactly straight down because the Earth is rotating. Someone rotating with the Earth must include the Coriolis force to predict the horizontal motion. Another example of a fictitious force associated with rotating reference frames is the centrifugal force.