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How do we measure velocity at the first place?

  1. Dec 9, 2003 #1

    jby

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    In time dilation and length contraction, there is a velocity term.
    How do we measure velocity at the first place?
     
  2. jcsd
  3. Dec 9, 2003 #2

    chroot

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    Relative to some other object, which you assume to be stationary.

    - Warren
     
  4. Dec 16, 2003 #3
    a bit closer to truth

    Actually, any reference frame is defined in a way, that in each point of space you have a spy - a being with a clock. Say an object is traveling in the space. Consider the following:
    first event the object passes the coordinate (x1,y1,z1) of the space. the spy sitting at that point remembers the time of this event: t1
    second event the object passes the coordinate (x2,y2,z2) of the space. the corresponding spy remembers the time of this event: t2
    Now, when t2->t1, the instant velocity is defined as:
    vx=(x2-x1)/(t2-t1);
    vy=(y2-y1)/(t2-t1);
    vz=(z2-z1)/(t2-t1);

    Practically, there are no spies rather there are clock associated with a reference frame, relatively to which you want to determine the velocity

    hope this answered your question :)
     
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