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Why does an object in motion remain in motion?

  1. Sep 29, 2004 #1
    Hello. I need an answer to a question that I couldn't find on the web.

    Why does Newton's first law hold? I mean is there an explanation why does an object in uniform motion tend to stay in this state of motion? Is this still a principle derived from mere observation or we can explain it with some underlying theory?

  2. jcsd
  3. Sep 29, 2004 #2


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    You left out the part about "unless acted upon by a force."

    Mere observation? Isn't that what science is all about - observation and theories? You don't get much more "underlying" than that. Besides, it makes perfect sense.
  4. Sep 29, 2004 #3


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    As of this date, there is no known "cause" for inertia. I believe some forms of string theory may be attempting to give a reason for this property of mass, but for the most part, it is simply accepted as a consequence of mass (in fact, a part of the very definition of mass). I recall reading that it was once thought that a spear thrown through the air caused a parting of the air, and this air rushing back in to fill the void is what kept the spear moving forward. Currently, there is no such mechanistic explanation.
  5. Sep 29, 2004 #4
    I don't mind if it is an observation. I just need to know how far the science has gotten.

    I am not sure if it makes perfect sense though. How does the object "know" where it has been a while ago, so that it can continue to move in the same direction and with same velocity? How does it "remember" that it is moving instead of being at rest? Do the objects have in fact some kind of "memories"?

    I would say that it would make much more sense if the objects move randomly instead of in such predictable fashion...
    Last edited: Sep 29, 2004
  6. Sep 29, 2004 #5
    No "known" cause

    Thanks LURCH, I guess that answers my question :smile:
  7. Sep 29, 2004 #6
    first law

    That a body in motion will remain in the same state of motion unless unbalanced forces act upon it is a matter of definition. What does it define? It defines inertial frames of reference. That is, inertial frame is a frame in which a body in motion will remain in the same state of motion unless unbalanced forces act upon it.

    The real law of nature as that it is possible to find such frames. So, your question can be reframed as - Why do inertial frames exist?

    The answer - symmetry. If it doesn't remain in the same state of motion, it must change the state, and there are an infinite number of ways in which that can happen and one can't be preferred over another.

    But there is just one way to remain in the same state of motion, and that is exactly what happens.

  8. Sep 29, 2004 #7
    WOW, that was a cool logic indeed, I like it, it makes sense :smile:
  9. Sep 29, 2004 #8


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    Another way to look at it is to take the other half of that law (that a body at rest tends to remain at rest), and apply it to moving bodies. This idea seems more intuitive to most people. Then Spacetime's point about inertial frames comes into play, when you aknowledge that motion is relative. So, a body in motion is a body at rest, to any observer in that body's inertial frame of reference (if you speed up to match the speed and direction of the object, it is "at rest" to you).
  10. Sep 29, 2004 #9


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    It is intuitive to look for causes that change things. If there is no cause, you can expect no change. And conversely, if there is no change in the behavior, you expect that there no agent acting on it.

    So, really, the answer should be "why not ?".
  11. Sep 29, 2004 #10
    "An object in motion stays in motion"
    in the reference frame of the moving object, this statement can be reformulated as
    "An object not in motion, stays in not motion"

    I realize the duck has been slaughtered, and while I'm against curelty to animals, it's really not that cruel to beat the duck if it is already dead (Although it is a sign one might be disturbed). So; to beat the dead duck (I also like hear my self talk, or write)

    "An object in motion stays in motion" Why? One of Einstein's posulates is that there is no prefered reference frame. So in the reference frame of a moving object, for arguments sake, let's say a duck, the duck doesn't always perceive himself as moving.

    Let me reforumulate the motion principle for the duck
    A: "A duck in motion, stays in motion"
    but wait, the duck can say "I'm standing still, you are the one who is moving"
    in which case statement A becomes
    "A duck thats not in motion, stays in not motion"

    So then the question is, how does the duck ever perceive himself moving?
  12. Sep 30, 2004 #11
    If you understand the Law of conservation of energy properly then it is much easier to understand why the Newton's first law holds. You see, L.C.E states that energy remained conserved for a proper system, now if any object have to move without any force acting on it then it voilates the laws of conservation of energy. Hence to increase the energy of the object we have to sped the energy on it i.e apply force on it.
    It might help you.
  13. Sep 30, 2004 #12

    Yes, I can see it in the context of the L.C.E. But still LCE is another principle derived from observation, so it seems that we are stuck here... like a dead duck :tongue2:

    In the context of LCE, I can reformulate my speculation like this - how does the system "know" what energy it has? But I guess I got the answer to that one -> it's a matter of cause and effect - if there is no cause, there is no effect :cool: For the system energy to change, there must be a force applied, so if there is no force, there is no change. But this is pretty much philosofical, I guess after time we will have a better understnding of the ways of the Universe :approve:
    Last edited: Sep 30, 2004
  14. Sep 30, 2004 #13
    it seems that you want a complete independent explaination for it. I think it is not so far posible to give you know that all the laws, theories are derived by observing the phenomena therefor what we derive is only what we observe and in my point of view explaining something like that is not possible only by simply observing. And i agree on that we can understand thing better when we start understanding universe better.
  15. Sep 30, 2004 #14
    So if in the reference frame of an object
    Newtons first law is reformulated as

    "An object not in motion does not start to spontaneously move"

    Q: Why should an object not in motion, not start to move?
    A: If the object did start to move, in which direction would it move?(<- this is a typical symetry argument)

    BTW; I used the wrong idiom. It's supposed to be "beating a dead horse". But I think duck works fine.
  16. Oct 1, 2004 #15
    It is matter of personal opinion, I think, but I do not like stating the first law like
    "An object not in motion does not start moving spontaneously" (1)
    What about an object that IS in motion?

    So, first law defines inertial frame. Perhaps we should even be more broad. Inertial frames are frames in which all laws of physics are equivalent. In this case the law:
    dp/dt = F_external is true. As far as we know, all forces are interaction forces.

    Imagine someone confined in an accelerating elevator and has no way to communicating with the outside. He would develope physics a lot differently. He found that objects in his world likes to go downward (toward the floor) and it is NOT due to ineraction with anything in his world. So he concludes that his world is anisotropic. Momentum and energy and such are not conserved. And his equation of motion would read: ma=F_interaction
    + F_floor.
    (j/k: perhaps the guy can invent a theory of gravity with massive floor?)

    So, I think a good definition of inertial frame is as follow:
    Inertial Frame are frames in which the world is (or remains) homogeneous and isotropic. And it is precisely the isotropicity that would demand "An object at rest stays at rest" . And also accelerated frame then cannot be inertial frame by definition.

    I don't think the object knows. In its frame, it's merely staying at rest. Since both frames (yours and the objects) are inertial, you can't prefer one frame over another (principle of relativity). Since your frame in inertial, object can't change its motion spontaneously, otherwise there is a preferred direction in your frame (the direction in which the object's motion changed without interaction).

    I wonder if one can argue that perhaps the frame can still be inertial (hence isotropic), if an object that spontaneous changes its direction at one point implies an opposite change to another object somewhere else. I suppose homogeneity and isotropic-ness must be locally true as well...

    another way of looking at this: you can certianly (classically anyway) plot x(t) in your frame. But it can only be of the form x(t)=x0+v0*t. There can be no acceleration or higher term, otherwise, you can conclude that the object is no in an inertial frame.

    A comment on a point of the first post. Observation is the BEST verification that our models and laws of the world are actually correct. A model with well observational support is a theory, those without are hypothesis (string?). Charge quanization is a good example. People has done dilute hydrogen gas exp. for long time hoping to see some charge different between proton and electron. Within experimental error (~2x10^-20e), none has every been observed. On the other hand, I wonder what is (if there is at all) the best experiment data on this topic.
    Last edited: Oct 1, 2004
  17. Oct 1, 2004 #16


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    There's a very simple way to look at all of this -- alluded to above by spacetime and mathfeel. The big one is the second law: if motion changes, then that change has a cause, which we call a force. Logic then says that no force no change. Nature agrees.

    2nd Law. Definition? Postulate? People have argued about this for a long time; no resolution in sight. But, one thing we can all say about the 2nd Law: inspired.

    No force, no change of motion. Pure Newton.
    Reilly Atkinson
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