Closing speed vs. velocity



As used in (relativistic) physics, the closing (or separating etc) speed is the rate of change of DISTANCE between TWO 'objects', as measured in some frame of reference.

(Relative) velocity is the rate of change of POSITION of a SINGLE 'object', as measured in some frame of reference.

These two different concepts happen to have the same value in classical physics (where the terms are then also often used interchangeably), but not necessarily in relativistic physics (where the distinction is important).

Sometimes the phrasing can seem confusing or ambiguous to some .. e.g. "velocity of A relative to B as measured in C", however, this form is clearing describing an opening or closing speed (the clue being the 'as measured in' referring to a frame of reference in which neither A nor B is at rest). Hence, "velocity of A relative to B as measured in C" is not the same concept as "velocity of A relative to B" or "velocity of A as measured in B" etc

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