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Paused physical system

  1. Aug 23, 2013 #1


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    "Paused" physical system

    Greetings, physicists and others, who just happen to understand physics. Something is bothering my mind again and I am hoping that some clarity will be given here.
    So, here is what I am thinking: if I Pause any physical system and, a bit after, sort of press "play" to continue the system - how does the system "know" which way to move?
    For example, for a simple ball thrown in atmosphere there could be, say, back and front air pressure difference, that can provide the direction information .. but if you start the system with just that information, the ball will actually move the opposite direction, because compressed air in front will tend to push the ball in the opposite direction. There must be more information and actual physical reason for the ball to move in the original way instead some other, random, way.
    In outer space there could be.. hmm.. blueshift and redshift, which can give you information about the direction and speed of the ball, but what is the reason for the ball to continue moving the way it did?
    I guess I am trying to understand inertia, because gravitational (and other) body attraction will continue after the system is resumed due to .. gravity (or EM or other). But even then - how would the system "know" the attracting bodies velocity before "paused"?

    And by "pausing" the system I dont mean doing anything mystical to the universe, but more like .. looking, measuring a moving system at any given point in time.
    Hope you understand.
  2. jcsd
  3. Aug 23, 2013 #2


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    Staff Emeritus
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    We can't "pause" the system. Objects in real life obey certain rules such as the conservation of momentum and cannot change directions on a whim.
  4. Aug 23, 2013 #3


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    And by "pausing" the system I dont mean doing anything mystical to the universe, but more like .. looking, measuring a moving system at any given point in time.
    Hope you understand.
  5. Aug 23, 2013 #4


    Staff: Mentor

    The state of a system is not specified only by it's (generalized) coordinates, but also by it's velocities. If you were to "pause" it you would be setting the velocities to 0, which would not be the same system. If you wanted to resume the same system, you would have to restore the original velocities.
  6. Aug 23, 2013 #5
    You could compare your thought experiment to selecting a single frame in a multiple frame animation sequence of frames, such as in a fluid simulation using some 3D modelling and animation software. Freeze framing the pouring of water into a glass, for example. Now press play, and see what happens..
  7. Aug 24, 2013 #6


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    and what will happen?
  8. Aug 24, 2013 #7
    The animation will continue until the last frame, of course. In any event, I don't see how this can contribute to your understanding of inertia, which is in itself rather self-explanatory. However, this might help: Remember Galileo's gravity experiment, in which he discovered that no matter what the mass of an object, it will always fall to Earth at the same rate, with an increase in velocity of 32 feet per second every second? It is because of inertia, which is described as the tendency for an object to resist a change in its motion. The heavier the object, the greater that resistance to change is, challenging the intuitive perception that it should fall faster because it is heavier, so how fast a body falls to the surface of the planet is a function of the mass of the planet, not of the falling object.
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