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Time of acceleration dissipation

  1. Jun 28, 2012 #1
    Trying to put aside, somehow, questions of body elasticity,

    How long does it take on average for a body, in outer space, to start moving at constant speed, from the moment that force has stopped acting on it?

    Or is this question, totally dependent on elasticity, and if you don't take elasticity into account, then the body could be considered as immediately stopping to accelerate when force stops acting on it?
    Last edited: Jun 28, 2012
  2. jcsd
  3. Jun 28, 2012 #2
    Rule of thumb is that force propagates in rigid bodies at the speed of sound in the respective material. So, if you apply a force at the rear of a rocket of length [itex]L[/itex] and the speed of sound in the rocket material is [itex]s[/itex], then the force will manifest itself at the nose of the rocket after [itex]t=\frac{L}{s}[/itex].
  4. Jun 28, 2012 #3


    Staff: Mentor

    Yes; for an idealized "point-like" body with no internal structure, it stops accelerating as soon as the force stops acting. As GAsahi said, when the body has internal structure (or when we have to include its internal structure in our model of the problem), then it takes some time for the information that the force has stopped to propagate through the body.
  5. Jun 29, 2012 #4
    Say that one day, there will exist a method to identify the amount and direction that forces were applied, on any given particle, since the time of the big bang, just like the rings inside a trunk of an old tree.. Would such a situation be exactly the same as saying, that ether was discovered? In other words, was ether always imagined as something that resides outside material? In other words, if a hypothetical correction to the Galilean principal of relativity is possible, as a result of a discovery of an intrinsic property of matter itself, would that be impossible, just on the same basis, that an ether external to material is impossible? In other words, could it ever be possible, that the impossibility of the existence of ether, has nothing to do with the possibility of corrections to the Galilean principal of relativity, as a result of properties, intrinsic to matter?
    Last edited: Jun 29, 2012
  6. Jun 29, 2012 #5


    Staff: Mentor

    whosapopstar, you appear to think that all of the questions you posed in your last post are just different ways of asking the same question. I'm not sure that's true.

    One issue is that different people mean different things by the term "aether", so it's hard to ask questions about it that are well-defined. The only one of your questions that I can really respond to is the first; I'm not sure what the others mean.

    As far as knowing the exact forces applied on a given particle since the Big Bang, if we could have such knowledge (which we can't, but I assume we're talking "in principle" here), which would be equivalent to exact knowledge of a given particle's worldline since the Big Bang, that knowledge could be gained and expressed and understood without ever requiring the concept of an "aether" as something independent of the rest of the dynamics. We can describe the worldlines of particles since the Big Bang perfectly well within the framework of General Relativity, which includes the dynamics of spacetime as part of the dynamics of the universe.

    Some people say that spacetime in GR counts as an "aether" because it's not really a material object, but if so, that means "aether" can be dynamical, which is not compatible with many people's idea of an "aether". So I don't know if the above really answers your first question or not. But the above is the basic physics involved.
  7. Jun 29, 2012 #6
    Yes, but when two forces are applied exactly once against the other (180 degrees), then they cancel each other, and one cannot trace them to the past (using current knowledge), or isn't it so? or even when two forces are applied one after the other, at different or even at the same direction, then you get a new situation, only with one direction, and cannot trace back the two seperate forces (amount and direction), that created the present situation of the particle, right?
    Last edited: Jun 29, 2012
  8. Jun 29, 2012 #7


    Staff: Mentor

    Forces may cancel on a particular object but still be "traceable" if they have other effects. But in general, as I said, we have no practical way of getting exact knowledge of an object's worldline far into the past even if particular pairs of forces do not cancel. Our observations of things far away and long ago are way too coarse-grained.

    I'm not sure what this has to do with your other questions, though.

    Edit: saw additions to your post, the above comments still apply.
  9. Sep 3, 2012 #8
    Hello Peter, and anyone else, who might help me understand questions, that pop up in my mind, and are probably situated somewhere between science, history of science, philosophy of science and science fiction.

    Let us assume something, much more crude and simple than the 'method' or 'measuring equipment', described in post #4 above. Let us assume that, in the near future, we will be able to tell, if an object floating in outer space, at a constant speed, had been accelerated by force, since one hour ago (one hour back), at a precision of, say, up to 0.00001 seconds. On a very general level, it is not without precedent, to be able to trace back a physical or chemical process, up to this level of accuracy, one hour back, isn't that so?

    Now, my question is, how would such an imaginary measuring equipment, stand in relation to the Galilean principle of relativity? Is the Galilean principle of relativity, a binary abstract model? What is it (a physical principal) exactly, in the very general mode? A mathematical object or, a philosophical object, or a logical object? What would be the difference? Is it a thing that is, either right or wrong, but never in the middle way? If the Galilean principle of relativity is a binary model, then the conclusion would be, that even a much cruder measuring machine (e.g. the one i suggest here), much cruder, than the one described in post #4 above, would negate its (the Galilean principle of relativity) validity, isn't that so?

    And, to keep on this line of thought (which is probably, even hypothetically, already flawed, some lines above), how does a concept of a physical principle, in general, and specifically, the Galilean principle of relativity, work? If it is only either right or wrong, what would negating it, even by a crude and short reaching method, change in modern physics and specifically in SR? If a physical concept, is not a binary thing, what would such an imaginary crude device, as i suggest here, change in modern physics and SR?

    I ask all this, because several times, i was told that SR works according to a postulate, that actually is derived from the Galilean principle of relativity (yes, yes and it is written in the first page of the 1905 paper etc...), but this discussion, as much as i think that i understand it, never goes beyond these details, and i think, as you read above, that i am missing many more details on this point.
    Last edited: Sep 3, 2012
  10. Sep 3, 2012 #9


    Staff: Mentor

    What do you mean by "accelerated by force"? In particular, are you thinking of gravity as a "force"? I suspect that you are, and if so, that is probably going to cause confusion for you.

    In GR, gravity is *not* a force; a "force" in GR, properly speaking, is something that causes an object to experience a nonzero proper acceleration--another way of putting it is that the object feels weight. Objects moving solely under the influence of gravity feel no weight--they are weightless, and they have zero proper acceleration.

    Objects moving solely under the influence of gravity can appear to "accelerate", to observers that are *not* moving solely under the influence of gravity. For example, the astronauts who dropped objects on the Moon (I use the Moon instead of the Earth to eliminate air resistance, which can confuse things) saw those objects appear to "accelerate" downwards; but actually, GR says that it was the astronauts who were accelerated--they felt weight, because the Moon's surface was pushing up on them, keeping them from falling freely. The dropped objects, while they were falling, were weightless, just like objects orbiting the Earth, for example in the Space Shuttle.

    I bring all this up because when you talk about measuring whether objects floating in outer space are "accelerated by a force", you need to specify whether you mean "accelerated" in the GR sense I gave above (accelerated == not weightless), or in the "apparent" sense in which a dropped rock is "accelerated".

    There is indeed a principle called the Galilean principle of relativity which is also one of the basic postulates of SR (it's not really "derived from" the Galilean principle, it's the same principle). That principle says, briefly, that unaccelerated motion (where "unaccelerated" is used in the above sense; i.e., it means weightless, as above) cannot be distinguished, locally, from rest (where "locally" means "using measurements made in a small region centered on the observer--the examples below should help to clarify that).

    Galileo used the example of a person inside a ship traveling at a constant speed across the ocean; purely from measurements made inside his cabin, he can't tell the ship is moving. A much better example nowadays is a person inside a spaceship floating freely (i.e., the ship and everything in it is weightless); purely from measurements made inside the ship, the person can't tell whether the ship is moving, or if so, at what speed.
  11. Sep 4, 2012 #10
    I was thinking of a force which is not gravity (would that be called 'non-zero proper acceleration'?), for example the force that a rocket engine produce, in a section of space, where gravity can be considered negligible, how does this change the answer?
    Last edited: Sep 4, 2012
  12. Sep 4, 2012 #11


    Staff: Mentor

    Ok, good, that means you are using the word "force" the way that it is supposed to be used in relativity.

    It doesn't; it means you are defining "unaccelerated motion" correctly, the way that it is supposed to be defined according to the principle of relativity. With that definition, the principle of relativity holds: locally, you can't distinguished unaccelerated motion from rest. If we detect that some object was accelerated, that doesn't invalidate the principle of relativity, because if the object is accelerated, its motion is no longer unaccelerated. :wink: If an object is accelerated, its motion *can* be distinguished from rest, because the acceleration is a direct local observable: you can measure it, locally, with an accelerometer (which basically measures whether the object feels weight, and if so how much).
  13. Sep 4, 2012 #12
    Yes, but please try to recall, that this is an imaginary situation, that i am trying to bring up here, in order to understand things, not necessarily from a realistic point of view, but rather as a more fictional 'what if' scenario.

    What i am trying to imagine here, is that in a future scenario, science will discover that matter changes its inner structure, as a result of proper acceleration, and keeps this new structure, even when that proper acceleration is over, without losing its energetic equilibrium (keeps this structure permanently, or at least for a duration that is much slower, than the speed of sound. You may consider this, a yet unknown 'super long lasting elastic' property, or something different, a permanent plastic property, that has to do with particles, and not with an external matter structure). Is such an imaginary scenario, in conflict, only with existing experimental results, or also with some other physical principal, besides the Galilean principle of relativity?
    Last edited: Sep 4, 2012
  14. Sep 4, 2012 #13


    Staff: Mentor

    I'm not sure I understand what "without losing its energetic equilibrium" means. So I'm not sure I can answer your question as you state it.

    But as far as the principle of relativity is concerned, the only thing that matters is whether the object's motion is unaccelerated. If it is, the principle of relativity says that locally, that motion can't be distinguished from rest. If it is not (i.e., if the object is accelerated), the principle of relativity doesn't really say anything about it at all.
  15. Sep 4, 2012 #14


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    It seems that you are proposing that if we had a complete "acceleration record" of an object, we might be able to determine its "absolute velocity" or trace its history back to when it was at absolute rest.

    If so, this is not the case. Even if we had such a record, we could not know anything about the object's absolute velocity, because we cannot just assume that the object was at some state of absolute rest at the start of the record.
  16. Sep 6, 2012 #15
    If it is possible, please see the rest of the thread, after the first post above, and let me understand, how will the Gelilean principle of relativity, should be regarded, in case that in the future, by testing an object material, and without using an accelometer, it would be possible to tell this very basic data: If an object was accelerated, a couple of hours ago (without knowing acceleration direction and without having any knowledge of this type, that is older than a couple of hours ago). It seems to me, that even a relatively 'crude' method, like this suggested here, would negate the Galilean principle of relativity, the way this principle is defined today. If this is not the case, please explain to me why it is not so.
  17. Sep 6, 2012 #16


    Staff: Mentor

    It's not so because, as I said before, the principle of relativity doesn't say anything about accelerated motion; it only says something about unaccelerated motion. In particular, the principle of relativity does not say that accelerated motion can't exist. So I don't see why you would think that discovering the existence of accelerated motion would negate the principle of relativity, which is basically what you appear to be saying.
  18. Sep 6, 2012 #17

    Gelilean principle of relativity is defined in such a way, as much as i understand it, that it says that if i want, i may check out with any existing experimental measuring device, an object body of matter, before it was accelerated (not while it is accelerated and certainly not with an accelometer), and even then ('even': no matter what measuring device i use), i will not be able to say, if it was accelerated afterwards (if a force was acting on it, creating a non-zero proper acceleration) - 'afterwards' means testing the object body some while afterwards, when it is yet (if was not accelerated), or again (if was accelerated) - moving at constant speed afterwards, checking it a second time: checking object body of matter again, with any measuring device i want (not while it is accelerated) - and will not be able to detect this information (if it was accelerated or not), using this second measurement, and comparing it to the first one.

    Next stage of my question is, imagining a fictitious scenario, where suddenly i am able to detect this kind of e data, using an imaginary experimental measuring device, that is able to detect if the object was accelerated in the past or not, without having any measuring data from the accelerating period itself.

    What is important for me to keep clear is, that this is not a plastic deformity of the object external structure, that i am referring to, with this imaginary measuring device, but some sort of imaginary particle bonding or particle radiation, 'yet unknown', that can be detected even when the plastic deformity of an object, as a result of that force acting, is absolutely negligible and undetectable.

    What i understand from you answers, peter, is that you rather answer only questions that arise from known experimental facts, and not from imaginary scenarios, that is very fine with me, if this is what you mean by ' as I said before', if so, you have given me a full answer, thanks.

    But, for anyone else, who may consider imaginary scenarios, i am asking this question, in order to see, how well i have understood the Galilean principle of relativity, since, as much as i understand this principle, if this imaginary measuring machine would have existed, even a very simple version of it, that can tell me that a body, although not deformed at any detectable external level, had gone through an acceleration period up to one hour ago, than the Galilean principle would be negated.

    If the answer is yes, i am right, that in such an imaginary scenario, the Galilean principle of relativity would be negated, than i may say that i think i understood that principle.

    But, if, even under such imaginary conditions, Galilean principle of relativity still holds, than i definitely did not understand this principle, or did not understand other physical principles, that still hold under this imaginary scenario or that make such an imaginary scenario impossible, and would like to understand, how, under these imaginary conditions, this/these principles would still hold.
    Last edited: Sep 6, 2012
  19. Sep 6, 2012 #18


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    You seem to be asking how can the known laws of physics be compatible with some imaginary device that operates on some imaginary and inexplicable principle that you have made up yourself. How can anyone answer that?
  20. Sep 6, 2012 #19


    Staff: Mentor

    I'm not sure I understand what you're trying to say here; I think your use of the word "acceleration" is causing confusion, because you're not actually concerned with the motion of the object while it's accelerating, but only before and after. Let me try to restate what I think you may be saying:

    Suppose we have an object that, at some time, is unaccelerated, and we measure it to be so. The principle of relativity says that this object's motion can't be distinguished locally from rest; in other words, the object has no unique "velocity" in any absolute sense. It only has velocity relative to other objects.

    Now we stop observing the object for a period of time, and during this period, it is accelerated by some non-gravitational force, so it experiences nonzero proper acceleration. But we have no measurements of that acceleration. After accelerating for a while, it stops accelerating and its motion becomes unaccelerated again.

    Then, some time later, we observe the object again, and again we find that its motion is unaccelerated and can't be distinguished from rest. So again, by the principle of relativity, the object has no unique velocity in an absolute sense, it only has a velocity relative to other objects.

    You appear to think that the above implies that the principle of relativity says that there is *no* way we can detect the change in the object's motion while it was accelerated, if we didn't actually observe the acceleration. That would imply that, if we somehow *were* able to detect, later, that the acceleration happened, without observing it directly, the principle of relativity would somehow be violated.

    But that's not true at all; there are plenty of ways we could detect that the object must have been accelerated when we weren't observing it. The simplest would be to put something else in the same state of motion as the object during the initial period (before it was accelerated), and then make sure that the something else was *not* accelerated. Then, when we started observing the object again, we would find that it was now moving relative to the something else, when it had been at rest relative to it before. That would show that the object must have accelerated while we weren't observing it.

    The principle of relativity in no way prevents our doing something like I just described; it does not say we can't distinguish *relative* motion from *relative* rest. Of course we can.
  21. Sep 7, 2012 #20
    Peter and Greg,
    I absolutely agree with your claims, on a up-to-date-realist thinking basis.

    P.S to my last post:
    "....that this is not a plastic **(or elastic)** deformity of the object...."
  22. Dec 18, 2012 #21
    If acceleration is considered roughly, as moving through a body (at outer space with no gravity) at the speed of sound, is there any reason that when it (acceleration) gets to the other edge of the body, it will not turn back towards the first edge, still at the speed of sound (but weaker), just as sound does?

    If it can be considered as turning back, then when exactly is it considered that detectable acceleration is no longer possible? Going back and forth through the body 2 times? 2000 times?... since if the accelerating force has stopped at one edge, then one wave through the body seems to make sense, as formal consideration of the end of acceleration, but then again, if acceleration still goes on bouncing between edges, it is not very clear when could it be considered to have stopped....

    Am i making any sense with this question, so i can be corrected at making wrong assumptions?
  23. Dec 18, 2012 #22


    Staff: Mentor

    It looks to me like you're again doing what DrGreg pointed out in a prior post: you're making up your own definition of "acceleration" and then trying to get answers to questions about it.

    You don't get to define what "acceleration" is; it's already defined. It means feeling weight; it means a nonzero reading on an accelerometer you carry with you. That's what it means. If you can't pose a question based on that definition, then you aren't asking about acceleration; you're asking about something else. I don't really understand what you're asking about, so I can't really answer.
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