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Determine whether both plane are in motion

  1. Jun 12, 2013 #1
    Suppose in deep space there are only two plane and no other reference is available. And each plane see other plane in motion.

    It is obvious that atleast one plane is in motion. But it is also possible that both plane are also in motion.

    Can the observer in these plane, tell whether their own plane too, is also in motion.

    Because to calculate other plane speed, an observer should know first whether his own plane is stationary or not.

    Last edited: Jun 12, 2013
  2. jcsd
  3. Jun 12, 2013 #2


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    OK, keep this all-important stipulation in mind when understanding the rest of your thread.

    OK, now you have a second stipulation.

    Yes, based on your two stipulations, you said that each plane can be a reference so in the rest frame of each plane, the other plane is in motion.

    Not according to your first stipulation, you said that no other reference is available other than the two planes.

    No, not according to your two stipulations, you said each plane sees the other one in motion and there are no other references so each plane must see itself at rest in its own reference.

    No, because in Special Relativity, when we calculate the speed of any object, it is always according to a reference frame. And you stipulated that your two planes were the only references available. They are quite adequate, no other reference is necessary.

    You're welcome.
  4. Jun 12, 2013 #3


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    You seem to be ignoring the basic concept of relativity: an object is in "motion" only relative to something else. It makes no sense to ask if something is in motion without specifying what object (or reference frame) relative to which it is in motion.

    That is why asking "Can the observer in these plane, tell whether their own plane too, is also in motion" is meaningless- you have not said "in motion relative to" a specific reference.

    Here, since there are only two objects, there are only two reference frames- and so two answers. Each passenger on a plane sees his own plane as stationary relative to itself and in motion relative to the other plane.
  5. Jun 16, 2013 #4

    If there is no other reference available or say no other reference is visible to a plane. And it accelerate, can we consider ourselves moving i.e we can always feel the acceleration.

    That is to say, I might be in motion with some unseen reference, but I am not aware of it. Or say when I am lost in deep space.
  6. Jun 16, 2013 #5


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    In a thought problem, if you started out inertial, you could use yourself as the basis for an Inertial Reference Frame (IRF) in which you are at rest. Then you could accelerate and now you would be moving in the same IRF. So if I understand your question, the answer is yes.
    You could start with the same IRF as defined earlier and then without accelerating at all, just use the Lorentz Transformation process to create a new frame moving at some speed with respect to the original IRF and now you have the object moving at some speed in a particlar IRF>
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