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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. 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 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. pulling objects away from rotating reference frames but the actual cause is the observer accelerating toward the axis of rotation. 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. Viewed from a general relativity theory perspective, appearing inertial forces (the supplementary external causes) are attributed to geodesic motion in spacetime. Introduction The motion of a body can only be described relative to something else—other bodies, observers, or a set of spacetime coordinates. These are called frames of reference. According to the first postulate of special relativity, all physical laws take their simplest form in an inertial frame, and there exist multiple inertial frames interrelated by uniform translation: Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K. This simplicity manifests itself in that inertial frames have self-contained physics without the need for external causes, while physics in non-inertial frames have external causes. The principle of simplicity can be used within Newtonian physics as well as in special relativity: The laws of Newtonian mechanics do not always hold in their simplest form...If, for instance, an observer is placed on a disc rotating relative to the earth, he/she will sense a 'force' pushing him/her toward the periphery of the disc, which is not caused by any interaction with other bodies. Here, the acceleration is not the consequence of the usual force, but of the so-called inertial force. Newton's laws hold in their simplest form only in a family of reference frames, called inertial frames. This fact represents the essence of the Galilean principle of relativity:   The laws of mechanics have the same form in all inertial frames. However, this definition of inertial frames is understood to apply in the Newtonian realm and ignores relativistic effects. In practical terms, the equivalence of inertial reference frames means that scientists within a box moving with a constant absolute velocity cannot determine this velocity by any experiment. Otherwise, the differences would set up an absolute standard reference frame. According to this definition, supplemented with the constancy of the speed of light, inertial frames of reference transform among themselves according to the Poincaré group of symmetry transformations, of which the Lorentz transformations are a subgroup. In Newtonian mechanics, inertial frames of reference are related by the Galilean group of symmetries. Newton's inertial frame of reference Absolute space Newton posited an absolute space considered well-approximated by a frame of reference stationary relative to the fixed stars. An inertial frame was then one in uniform translation relative to absolute space. However, some "relativists", even at the time of Newton, felt that absolute space was a defect of the formulation, and should be replaced. The expression inertial frame of reference (German: Inertialsystem) was coined by Ludwig Lange in 1885, to replace Newton's definitions of "absolute space and time" by a more operational definition. As translated by Harald Iro, Lange proposed the following definition: A reference frame in which a mass point thrown from the same point in three different (non co-planar) directions follows rectilinear paths each time it is thrown, is called an inertial frame. The inadequacy of the notion of "absolute space" in Newtonian mechanics is spelled out by Blagojević: The existence of absolute space contradicts the internal logic of classical mechanics since, according to the Galilean principle of relativity, none of the inertial frames can be singled out. Absolute space does not explain inertial forces since they are related to acceleration with respect to any one of the inertial frames. Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon. The utility of operational definitions was carried much further in the special theory of relativity. Some historical background including Lange's definition is provided by DiSalle, who says in summary: The original question, "relative to what frame of reference do the laws of motion hold?" is revealed to be wrongly posed. For the laws of motion essentially determine a class of reference frames, and (in principle) a procedure for constructing them. Newtonian mechanics Classical theories that use the Galilean transformation postulate the equivalence of all inertial reference frames. The Galilean transformation transforms coordinates from one inertial reference frame, s {\displaystyle \mathbf {s} } , to another, s ′ {\displaystyle \mathbf {s} ^{\prime }} , by simple addition or subtraction of coordinates: r ′ .... Discover the Amedeo Balbi popular books. Find the top 100 most popular Amedeo Balbi books.