Editing Inertial frame of reference (section)

{{Short description|Fundamental concept of classical mechanics}}
{{Use dmy dates|date=August 2019}}
{{Classical mechanics|cTopic=Core topics}}

In [[classical physics]] and [[special relativity]], an '''inertial frame of reference''' (also called an '''inertial space''' or a '''Galilean reference frame''') is a [[frame of reference]] in which objects exhibit [[inertia]]: they remain at rest or in uniform motion relative to the frame until acted upon by external forces. In such a frame, the laws of nature can be observed without the need to correct for acceleration.

All frames of reference with zero acceleration are in a state of constant [[rectilinear motion]] (straight-line motion) with respect to one another. In such a frame, an object with zero [[net force]] acting on it, is perceived to move with a constant [[velocity]], or, equivalently, [[Newton's laws of motion#Newton's first law|Newton's first law of motion]] holds. Such frames are known as inertial. Some physicists, like [[Isaac Newton]], originally thought that one of these frames was absolute — the one approximated by the [[fixed stars]]. However, this is not required for the definition, and it is now known that those stars are in fact moving, relative to one another.

According to the [[Principle of relativity#Special principle of relativity|principle of special relativity]], all [[physical laws]] look the same in all inertial reference frames, and no inertial frame is privileged over another. [[Measurement|Measurements]] of objects in one inertial frame can be converted to measurements in another by a simple transformation — the [[Galilean transformation]] in [[Newtonian physics]] or the [[Lorentz transformation]] (combined with a translation) in [[special relativity]]; these approximately match when the relative speed of the frames is low, but differ as it approaches the [[speed of light]].

By contrast, a ''[[non-inertial reference frame]]'' is accelerating. In such a frame, the interactions between [[physical object]]s vary depending on the acceleration of that frame with respect to an inertial frame. Viewed from the perspective of [[classical mechanics]] and [[special relativity]], the usual [[Fundamental forces|physical forces]] caused by the interaction of objects have to be supplemented by [[fictitious force]]s caused by [[inertia]].<ref name="Rothman">{{Cite book|title=Discovering the Natural Laws: The Experimental Basis of Physics |author= Milton A. Rothman |page=[https://archive.org/details/discoveringnatur0000roth/page/n37/mode/2up 23-24] |url=https://archive.org/details/discoveringnatur0000roth
|url-access=registration |quote=reference laws of physics. |isbn=0-486-26178-6 |publisher=Courier Dover Publications |date=1989}}</ref><ref name="Borowitz">{{Cite book|title=A Contemporary View of Elementary Physics |page=[https://archive.org/details/contemporaryview00boro/page/138 138] |publisher=McGraw-Hill |date=1968 |url=https://archive.org/details/contemporaryview00boro|url-access=registration |asin= B000GQB02A |author1=Sidney Borowitz |author2=Lawrence A. Bornstein }}</ref> 
Viewed from the perspective of [[General relativity|general relativity theory]], the fictitious (i.e. inertial) forces are attributed to [[geodesic (general relativity)|geodesic motion in spacetime]].

Due to [[Earth's rotation]], its surface is not an inertial frame of reference. The [[Coriolis effect]] can deflect certain forms of motion as seen from [[Earth]], and the [[centrifugal force]] will reduce the effective [[gravity]] at the [[equator]]. Nevertheless, for many applications the Earth is an adequate [[approximation]] of an inertial reference frame.