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Frames of reference

  1. Jan 18, 2009 #1
    I keep hearing how motion only exists relative to something else. But what if there is some object in space, and it is the only object in the universe. It has some kind of propulsion system, and it activates it, goes to a certain speed, then stops accelerating. Is that object really moving, or since there is nothing to compare it to, then is it technically not moving? If it is not moving, where did its kinetic energy go?

    Another situation: What if you had only two objects in the universe. They are moving at the same speed in the exact same direction so are parallel to each other. Are they moving at all either?

    And now here raises another point. We all know that the earth is revolving around the sun, not the other way around. How can we know that as a fact since due to inertial frames, the sun and all the stars may just as well be rotating around the earth instead?

  2. jcsd
  3. Jan 18, 2009 #2


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    If the the object has a rocket engine, then it will have velocity relative to the expellant. In fact any propulsion system known to us involves matter other than the object.

    There's no way to define either objects velocity except wrt to the other object.

    rotating frames are not inertial, so the symmetry does not apply. Rotation always produces an acceleration that can be detected in the rotating frame.
  4. Jan 18, 2009 #3
    This is not an answer, but another question regarding frames of ref, so I thought I'd tack it on to the same thread.

    If there are three objects in space, myself a freind and a bungee rope between our legs.
    We start counting and kick off from each other at say C/3. When he returns to me I may expect his count to be lower than mine as from my frame has has made a round trip at high speed and returned to me, yet, he would expect the same from me?

    Both cannot be true, and I expect neither case is true, and that we will return on the same count, and, If there was a third person at the center of the bungee rope who also started counting, that his count would be higher than ours.

    If we could telepathically hear each other counting, I would hear the count of my kickoff freind stay the same as mine, even though we are moving apart at 2C/3, and I would hear the count of the central observer speed up, even though we are moving apart at C/3.

    This does not seem very relative to me, it imply's that there IS an absolute rest which does matter (the center of the bungee).

    Comments please?
  5. Jan 18, 2009 #4


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    It's only in inertial frames that moving clocks always run slow. In the bungee case the two of you don't move inertially (although perhaps the guy at the center does), you accelerate to turn around and begin returning to one another (inertial motion means constant speed and direction). See the twin paradox which is fairly similar to your example.
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