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Matter and Inertia

  1. Jun 15, 2004 #1
    Consider this thought experiment. If only one object existed in all the universe and it was moving would it have inertia?
  2. jcsd
  3. Jun 16, 2004 #2
    It has inertia regardless of whether or not it was moving. Inertia is not a function of speed.
  4. Jun 16, 2004 #3


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    If it was just one object, you would not be able to define movement. So the question is improper. A more interesting question is If there was one object plus yourself as observer, would both of you have inertia?
  5. Jun 16, 2004 #4
    The question is more about what is the cause of inertia.
  6. Jun 16, 2004 #5
    That is true but given the fact that you can't define motion does an object completely isolated have inertia?
  7. Jun 16, 2004 #6
    Inertia requires a frame of reference. If your frame of reference includes this object at rest, then the object will have no inertia in that frame. If your frame of reference has the object in motion, then the object has inertia in that frame.
  8. Jun 16, 2004 #7
    Isn't that describing momentum? I thought inertia and momentum were different things.

    Inertia is an object's resistance to a change in its state of motion. To my understanding, mass is a measure of inertia. If your object has mass, then it has inertia, whether it is at rest or not. Zero momentum is not equivalent to zero inertia.
  9. Jun 16, 2004 #8
    I'm sorry, you're right, for some reason I always have those two switched in my mind. The object will always have inertia regardless of whether it is in motion or not. Of course, without an understanding of our own universe, we would never know it has inertia because no external forces would act upon it. However, we know it would still have inertia because if some force WERE to suddenly act upon it, it would exert equal and opposite forces to oppose its acceleration.
    Last edited: Jun 16, 2004
  10. Jun 16, 2004 #9
    The question remains does the object I described have inertia?
  11. Jun 16, 2004 #10
    You have answered your own question. By isolating the object, you only ensure that no forces will act upon it. You are nullifying the effect of inertia because how can an object resist zero change? However, nullifying inertia does not eliminate inertia. If some force in the future should act upon that object, inertia will again spring into action. As long as the object has mass and is governed by physics as we know, it will always have inertia.

    Allow me to draw an analogy: If you slack all tension in a rubber-band and set it on a table, is the rubber-band still elastic?
  12. Jun 16, 2004 #11


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    If it has mass, it has inertia.
  13. Jun 16, 2004 #12
    If motion is not defined [for the object] how can inertia be defined?
  14. Jun 16, 2004 #13


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    If some internal reaction within the object causes it to divide into two or more parts, then the velocities of these parts (in the frame of the mother object - the bomb) is inversely related to their inertias. These parts could recombine (because they are oppositely charged ?) and re-form into the mother object, which then has velocity in the frame of one of the sub-parts.
  15. Jun 16, 2004 #14


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    It is not clear to me that Physics, which was created to model our multi-particle universe, has anything meaningful to say about such a non physical universe. Non Physical assumptions lead to no meaningful results.
  16. Jun 16, 2004 #15
    Stick to the topic there is nothing mentioned about dividing. Just answer the question.
  17. Jun 16, 2004 #16
    I understand and agree with your point. What I am trying to do is build up an idea based on this hypothetical situation that will eventually have some physical meaning.
  18. Jun 17, 2004 #17
    Why is motion a requisite for inertia? You said yourself that if the object has mass, it has inertia. This is the only requisite. Motion is merely a matter of the frame of reference. Inertia exists independant of a frame of reference. To say that an isolated object would not have inertia is to say that an isolated object would not have mass.

    I can tell you are trying to lead us somewhere with this and not having much luck. Why don't you just come out and say what you're trying to say?
  19. Jun 17, 2004 #18
    I'm not trying to lead you somewhere. I am trying to get a discussion going.

    Inertia is resistance to change in motion. If motion is not defined how can resistance to change in motion be defined?
  20. Jun 17, 2004 #19
    If no current is going through a wire, does it still have resistance?

    If a tree falls in the woods, and on one is around to hear it...
    Last edited: Jun 17, 2004
  21. Jun 17, 2004 #20
    You examples do not apply. You have confused current not being present in a particular place with the non-existence of current anywhere. Resistance still exists in the wire because you could apply a current to it. As far as the second example goes that is just a stupid question. If you are not going to ask adult questions then leave.
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