Inertial frame of reference

In <a href="/facts/Classical_physics/gGnINVPE">classical physics</a> and <a href="/facts/Special_relativity/vsFFc63E">special relativity</a>, an inertial frame of reference (also called an inertial space or a Galilean reference frame) is a <a href="/facts/Frame_of_reference/15yYfB9M">frame of reference</a> in which objects exhibit <a href="/facts/Inertia/bzK9uzI2">inertia</a>: 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 <a href="/facts/Rectilinear_motion/Vom0QwWe">rectilinear motion</a> (straight-line motion) with respect to one another. In such a frame, an object with zero <a href="/facts/Net_force/XzbmueXk">net force</a> acting on it, is perceived to move with a constant <a href="/facts/Velocity/Q3o1LVDe">velocity</a>, or, equivalently, <a href="/facts/Newton%2527s_laws_of_motion/XKzGTXK1">Newton's first law of motion</a> holds. Such frames are known as inertial. Some physicists, like <a href="/facts/Isaac_Newton/4L7gTncN">Isaac Newton</a>, originally thought that one of these frames was absolute — the one approximated by the <a href="/facts/Fixed_stars/grlDbIPX">fixed stars</a>. 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 <a href="/facts/Principle_of_relativity/8I2xlmoP">principle of special relativity</a>, all <a href="/facts/Physical_laws/aVtc4wpk">physical laws</a> look the same in all inertial reference frames, and no inertial frame is privileged over another. <a href="/facts/Measurement/rsLLbn1s">Measurements</a> of objects in one inertial frame can be converted to measurements in another by a simple transformation — the <a href="/facts/Galilean_transformation/CR8VWxbr">Galilean transformation</a> in <a href="/facts/Newtonian_physics/ykG8upsN">Newtonian physics</a> or the <a href="/facts/Lorentz_transformation/jotJL06G">Lorentz transformation</a> (combined with a translation) in <a href="/facts/Special_relativity/vsFFc63E">special relativity</a>; these approximately match when the relative speed of the frames is low, but differ as it approaches the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a>.
By contrast, a <a href="/facts/Non-inertial_reference_frame/UGfKCASo">non-inertial reference frame</a> is accelerating. In such a frame, the interactions between <a href="/facts/Physical_object/WhqjK6xo">physical objects</a> vary depending on the acceleration of that frame with respect to an inertial frame. Viewed from the perspective of <a href="/facts/Classical_mechanics/ykG8upsN">classical mechanics</a> and <a href="/facts/Special_relativity/vsFFc63E">special relativity</a>, the usual <a href="/facts/Fundamental_forces/WrFEJaFl">physical forces</a> caused by the interaction of objects have to be supplemented by <a href="/facts/Fictitious_force/nJp96HPc">fictitious forces</a> caused by <a href="/facts/Inertia/bzK9uzI2">inertia</a>. 
Viewed from the perspective of <a href="/facts/General_relativity/jVsoIg8X">general relativity theory</a>, the fictitious (i.e. inertial) forces are attributed to <a href="/facts/Geodesic_(general_relativity)/PEBX8FSm">geodesic motion in spacetime</a>.
Due to <a href="/facts/Earth%2527s_rotation/mDbXcKUx">Earth's rotation</a>, its surface is not an inertial frame of reference. The <a href="/facts/Coriolis_effect/SqCyUY6d">Coriolis effect</a> can deflect certain forms of motion as seen from <a href="/facts/Earth/HLLmDfj0">Earth</a>, and the <a href="/facts/Centrifugal_force/CWgvMw6w">centrifugal force</a> will reduce the effective <a href="/facts/Gravity/bqAN7DdQ">gravity</a> at the <a href="/facts/Equator/CjaxbVaN">equator</a>. Nevertheless, for many applications the Earth is an adequate <a href="/facts/Approximation/LvYHsXzq">approximation</a> of an inertial reference frame.

Inertial frame of reference open-in-new

Inertial frame of reference