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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Relevant Pages

  • Re: Time Dilation reduces the Speed of moving Objects
    ...    The physicality of Zeno's conclusion is wrong. ... the velocity remains the same. ... I notice you keep definding classical physics. ... frame, this does NOT mean that, unless we have a mental image of what ...
    (sci.physics.relativity)
  • Re: It is not always c!
    ... To calculate closing or separation velocity, ... Relative velocity is NOT the same thing as closing or seaparation speed. ... different time in the frame of reference of that ... from measurements, not measurements themselves. ...
    (sci.physics.relativity)
  • Re: It is not always c!
    ... velocity measurements can be made where there are two ... frame A and frame B as two valid SR frames. ... I assume what you REALLY mean is you are taking closing and separation ...
    (sci.physics.relativity)