= Inertial frame
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= Inertial frame of reference
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An <inertial frame> is a <reference frame> in which a free particle follows a straight path at constant <velocity>. This is the setting in which the usual form of <Newton's second law> applies without additional forces caused by acceleration of the frame itself.
Frames moving at constant relative velocity can both be inertial. In <Newtonian mechanics>, a <Galilean transformation> relates their spatial velocities while preserving absolute time; in <special relativity>, the corresponding relation is a <Lorentz transformation>.
A freely moving object viewed from an accelerating vehicle can acquire coordinate acceleration even when no new interaction acts on it. Apparent-force terms account for that frame motion. An inertial description is an idealization whose accuracy depends on the spatial and temporal scales of the experiment.
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