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How does an object know that it is in motion?

  1. Apr 4, 2008 #1
    How does an object "know" that it is in motion?

    As an object that you have thrown is flying through the air, how can it know that it has kinetic energy? If you "froze" time and looked at the object. What seperates it form a similair object without the motion energy? Where is this "information" stored? Is motion perhaps a kind of shape in the 4th dimension?
  2. jcsd
  3. Apr 4, 2008 #2


    Staff: Mentor

    It doesn't "know". KE is frame-dependent, it is not a property of an object.
  4. Apr 4, 2008 #3
    very well

    Still the "information" must be "stored" somewhere. The universe must have a mechanism for knowing what to do next. :approve:
  5. Apr 4, 2008 #4


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    Not only does it not know it's in motion, but even you don't know it's in motion, and there's nothing you can do to prove it.

    Meteors come plunging into our atmo all the time because they're whizzing all over the solar system. Earth is on the move too in its ornit.

    If a meteor were hovering in space minding its own business and the Earth came whizzing around the corner and smashed into the meteor, the meteor would still do a huge amount of damage to the Earth.

    The kinetic energy is due to the relative velocities between the two bodies, regardless of which is moving, not an intrinsic propertry of one object or another.
    Last edited: Apr 4, 2008
  6. Apr 4, 2008 #5
    If that object is a part of your computer simulation, then the information is stored in your computer's RAM or hard drive. For a real life object you better ask this question on a religious forum.
  7. Apr 4, 2008 #6
    way of speaking

    I am awere of this. I guess Im getting a bit missunderstood. I was speaking in pictures. I dont think objects have a conciousness or a will, and I am not religious. As the distance between two objects are increasing - what makes it continue to do that? there must be a deeper issue to the concept motion than that it just does or "they have a velocity relative to each other" or "they have KE"
  8. Apr 4, 2008 #7


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    Ah. The intrinsic property you're looking for is inertia - the tendency of an object in motion to remain in motion, and an object at rest to remain at rest.

    Described by Newton's First Law.
  9. Apr 4, 2008 #8
    If you filmed the ball in motion you would see slight blurring in each frame and you could guess the location of the ball in the next frame. You would need to look at 2 frames to see if the ball is accelerating to get an even better guess prediction of where the ball will be in the third frame. If the exposure time is speeded up the blurring is reduced but never entirely elliminated because that would require zero exposure time for each frame. One paper suggested that there is no such thing as an "instance" of time. There is a minimum time interval (possibly the Planck time interval?) which you can not go below.

    The uncertainty principle sort of supports this view. It states that the exact velocity and location of a particle is never completely defined. There is a sort of "fuzziness" whereby if you know the exact location of the particle then you can not be certain of its velocity (should that be momentum?) and if you know its velocity exactly then there is uncertainty about its location. Quantum mechanics supports this viewpoint and that is a very sucessful theory.
  10. Apr 4, 2008 #9
    No. It is not what im thinking of, though it is close but fare away, but Newtons first law after all just describes the behavior of objects. My focus is pretty far outside the box - but still into physics. I guess Im not able to make myself understood, and perhaps it is just crap anyway - so I guess the best is to end the thread. But my soul will never rest :)

    edit: kev, I find your answer very interesting, yet I dont completely understand it as I dont know QM, but I sure like the smell of what you perhaps told me :) Cheers!
    Last edited: Apr 4, 2008
  11. Apr 4, 2008 #10
    Objects that are in relative motion with each other have a different orientation in spacetime. Unless they change their orientation, by accelerating, their relative motion remains the same.
  12. Apr 4, 2008 #11

    D H

    Staff: Mentor

    This still doesn't explain the OP's question. The answer is that, in both classical and modern physics, momentum is a consequence of time, space, and mass. Time, space, and mass are axiomatic in classical physics. (Mass, maybe not, in modern physics, but time and space remain axioms). Why they exist, we don't know.
  13. Apr 4, 2008 #12
    Great question. I think i understand it somewhat but am more certain the answers posted do not answer it.

    you could ask how does a train know which direction to go. This comes close but the analogy fails b/c the train is driven by the engine. Yet, imagine a train in space on a frictionless track. The train, once in motion would be guided bt the track. This is analogous to spacetime determining the paths objects move on. What it does not explain is why the train keeps moving at a constant speed, i.e. It doesn't explain inertia.

    In the case of constant velocity you can't say if the train is moving or not, from the point of view of the train. You could not do an experiment to test this. So how do two people know they are in relative motion to each other without appealing to epistemological reasonings, i.e. B/c i see the train moving away. Here you would simply mark the position of the train at different times and see if it is changing. But this also involves an observation so we can not avoid some kind of appeal to the senses in the end.
  14. Apr 4, 2008 #13
    In relativity you cannot speak of space and time separately except for the experience of a particular observer and even in that case it is limited when spacetime is curved. For instance the notion of distance in static curved spacetimes for a particular observer is slightly problematic, in non static spacetimes the notion of distance is even more complicated.
  15. Apr 4, 2008 #14


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    How is the answer to your question not inertia?
  16. Apr 4, 2008 #15


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    One of the lessons from relativity (already at the Galilean level) is that there is no state of absolute rest. Thus, there is a reference frame [in fact, many such] in which the object is in motion.
  17. Apr 4, 2008 #16
    The rules about inertia is based on observations of objects. I simply dont think that interia is hard to understand, but what is going on in a deeper level? My guess is that the answer Im looking for involves somehow spacetime and more fundamental issues. We are so used to observe objects in motion, and as we are pretty used to predict moving objects behavior it is easy to fall in the trap to think that we actually know what motion is just by observing it and being able to predict it.
  18. Apr 4, 2008 #17


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    That is physics.
    But what makes you think there is a deeper level? What is it that you think has not been adequately explained?
  19. Apr 5, 2008 #18

    It is not sertain that there is anything deeper here, after all I just feel that it is has to be, and it is not easy to explain that. The only time I am successful in explaining it is when the listener accedentally allready have the same idea. As I see it that to say that motion is just velocity relative between objects and such things are quite OK but it is as just as satisfying as an answer to "what is gravity?" with "Gravity is objects falling to the ground" or "That every pair of objects have a attraction on eacg other depending on mass" or that "gravaty decreases with the squere of the distance" etc. None of those answers describes what causes gravity. What makes it tick. Perhaps Im taking it a bit far to say that motion is as mysterious as gravity (perhaps it is) but it is to make my point. To ever understand the answer to my question if there is one - i guess I have to start studying som physics anyway. The question was asked in plain curiousity.
  20. Apr 5, 2008 #19
    anthropic principle

    I partly understand where you are coming from. We have equations that describe the observations we make and come up with a set of rules that seem to make sense, but right deep down fundementaly we do not know "why" the universe behaves the way it does. Why the value of the fine structure constant is 1/37.03599911... is a mystery. Sure, we can say it comes from the relationship of elementary charge, Planck's constant, the speed of light and the permitivity of free space but then we have to explain why those other constants have the values they have, independently of the fine structure constant.

    One explanation given by scientists is the anthropic principle. They conjecture there many possible universes each with their own set of rules, but only some of those possible universes have rules that can allow life to evolve and prosper and only in those universes are there sentient beings that can ask the question "Why does this universe we find ourselves in, happen to have rules that allows us to exist?".
  21. Apr 5, 2008 #20
    Well, if by inertia you mean the whole "objects in motion stay in motion"... well, take an object in motion. Now, pick a reference frame in which it is instantaneously at rest. Assume that there are no other objects, and that the object you're observing is rotationally invariant. Ask yourself "in which direction will the object move?" If you come up with an answer then that answer had better not break rotational invariance.

    Now it may seem that I've just swept the question under a different rug, but you'll notice that symmetry plays an important role here. It's a mathematical observation that symmetrical systems have conserved quantities (c.f. Noether's theorem) so your question could be phrased "why is nature symmetric?". I might be tempted to answer "why not" but oh well =)
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