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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