= Reference frame
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A <reference frame> specifies how positions and times are assigned to physical events. It includes a choice of spatial axes and a procedure for relating events to time readings; different frames can describe the same event with different coordinates.
A frame attached to a laboratory is convenient for reporting apparatus measurements, while a <centre-of-momentum frame> can simplify collision kinematics. These are physical choices of motion, not merely different unit labels. Translating, rotating or accelerating the reference axes changes the form of the coordinate description.
An <inertial frame> allows force-free particles to move uniformly. In an accelerating or rotating frame, additional apparent-force terms enter the equations of motion. Comparing observations therefore requires transforming coordinates, velocities and forces consistently rather than changing only one quantity.
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