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Who is moving

  1. Apr 20, 2008 #1
    If i have two fish in a fish tank say fish A and fish B and the owner of the tank is looking into the tank

    each fish thinks he is travelling at some velocity.

    fish A looks at fish B and Fish A doest know if he is moving or if fish B is moving. Same for fish B. But the owner looking into the tank can absolutely tell which fish is moving.

    As I said in another post two frames of reference are always conatined within a third larger frame.

    can someone explain to me why the perspective of the fish is the preferred frame?

    the owner measures the fishs velocity and call him on this two way radio and tells him his velocity. Is this an absolute velocity?
  2. jcsd
  3. Apr 20, 2008 #2
    In your example, the owner would just happen to be moving as fast as the fish he thinks is not moving. What if the owner's friend was walking by the fish tank, walking as fast as the "moving" fish? Now take this into space. Which fish/person system is moving and which is not?
  4. Apr 20, 2008 #3

    This question should be moved to the forum on Newtonian Physics or Mechanical Engineering. Any engineer who designs moving machinery can answer it.
  5. Apr 20, 2008 #4


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    Staff: Mentor

    Indeed: who owns space?!

    Rab, we are quite clear on how you think the universe works. It doesn't work that way. There really isn't anything to be gained by repeating your misunderstanding over and over and over again. We get it, but it doesn't get any less wrong the more often you repeat it.

    Perhaps you learned something though: your fish tank example utilizes Galilean relativity (I mentioned this before). I'm not quite sure you're completely there, though: your fish tank is a "preferred" frame only insofar as it is useful for the owner of the tank to measure the speed of the fish with respect to the water in the tank. But it still doesn't necessarily make any sense for others to use that frame. And that's perfectly fine.
    Last edited: Apr 20, 2008
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