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Center of mass and motion

  1. May 10, 2007 #1
    fact - " the center of mass of an isolated system doesn't alter its position until an external force acts on it." ( i'm guessing its right)

    inexplicable(may be apparently) - " No body is pushing me. But i'm still moving"

    How do you explain that? Am i missing something?
  2. jcsd
  3. May 10, 2007 #2
    What are you talking about?
  4. May 10, 2007 #3
    Alright let me get technical.

    suppose this event :-

    A space vehicle is in a zero gravity place( assume). Now there is no external force acting on the space vehicle due to the absence of gravity and the space vehicle remains there. Even though there's no air around to work the Newtons third law out, we can make the vehicle move. Lets see how..

    Lets consider the whole space vehicle as one system. the center of mass of that system remains there no matter what( as there's no exernal force).But the passengers are sitting in the front part of the vehicle and they want to move forward. So they send out large amounts of combusted fuel (out of the butt of the vehicle) at high velocities. As a part of the system is reconfigured, the rest of the system should also reconfigure so that the center of mass of the entire system remains at the same position. This causes the body of the vehicle to move forward. That's the principle ( if i'm correct)

    Now my question( it's a thought experiment actually) - " suppose we're in that zero gravity place instead of that vehicle.....we can punch the space with our fist without the need for any external force....and that'll cause the rest of our body to move backward so that the center of mass of the entire body remains in the same position. But, when we come to rest after the impulse, we notice that our body( excluding the arm) has moved a distance away from the fist( though the distance is small when compared to that of what our fist has travelled).

    So, here comes my question - " When we tried to move from that spot(by punching the space), what initiated that process? why did we try to punch?"This is a very delicate question which can be easily misunderstood. so let me elaborate - "Like every physical effect has a physical cause preceding it, what was the preceding cause for that punch of the fist?" I believe that the actual reason for not having a future predicting mechanism is not heisenberg's uncertainty principle that said-"If we do not have accurate knowledge of the configuration of the present universe, then, how would we use the principles of interaction to determine the future of the universe accurately?" but the fact that we do not know the preceding physical cause for actions that we living beings do. Get it?

    I know, some of you would like to believe that i'm going off-track or i'm getting into the realms of philosophy.

    But consider this infamous yet sensible saying :-

    "Science is conclusions drawn from rational analysis of accurate observation. If every tangible and intangible phenomenon(caused by the chemicals and electrons flowing in our head) is a repercussion of existence and interaction of physical matter, which is observable, then why can’t science explain everything?"

    I hope physicists will one day realize this and probably then, we'll have a merging of psychology, physics, philosophy etc.,

  5. May 10, 2007 #4


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    Staff: Mentor

    When you pull your fist back to your body, you move back to your initial position. There is no such thing as a reactionless drive.
  6. May 10, 2007 #5
    That is exactly my argument! I don't see why you have posted this. I know the fact that - "the center of mass of an isolated system cannot change until external force acts upon it"

    Please read my message thoroughly.

  7. May 10, 2007 #6


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    The way you worded it, I didn't see that you understood that. So you do....good. I really don't see the point here, then. What are you asking? The rest of the post seems to be a philosophical musing on causality, but I don't see a relevant question anywhere there. Physics doesn't care why you chose to punch the air, it just explains what happens when you do.
  8. May 10, 2007 #7
    As a human being, aren't you curious as to why it happens?

    Anyway, do you think that every physical effect should have a physical cause?
  9. May 10, 2007 #8

    Doc Al

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    Staff: Mentor

    You guessed wrong: The center of mass of an isolated system continues to move in a straight line at constant speed unless acted upon by an external force. A net external force is only needed to change the velocity of the center of mass.
  10. May 10, 2007 #9
    I assumed that the isolated system must have been at rest with some frame of reference. Anyway, thanks for the minor correction.

    Mind joining the causality discussion and shedding some light on it for the cause of my poor perception of that subject?

    Last edited: May 10, 2007
  11. May 10, 2007 #10

    Doc Al

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    Staff: Mentor

    You had said...
    That's still thinking that one needs a force to "explain" motion. (You're missing Newton's First law.)
  12. May 10, 2007 #11
    Okay okay!!

    Yeah i get it! i just didn't represent my ideas thoroughly in my first post( i was hasty by the way).

    "When i move, the *ground* pushes me."

    But now, i'm asking - " What was the cause for my action? (i.e. pushing the ground to let it push me)
  13. May 10, 2007 #12


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    Staff Emeritus
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    Gold Member

    That isn't Newton's first law, that's his third. I suggest you look up Newton's first law.
  14. May 10, 2007 #13


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    That doesn't make any sense to me. Cause and effect isn't really a "why" question.

    Cause: you moved your fist
    Effect: your body moved in the opposite direction.

    Why you moved your fist in the first place (if that is what you are asking) isn't a physics question, it is a question of motivation or philosophy. This whole line of questioning doesn't make much sense or seem very relevant to me.
  15. May 10, 2007 #14
    I think I understand what you're asking. Are you wondering why you decided to push the ground?
  16. May 10, 2007 #15

    To be more precise, Lets assume that At t=0, i pushed he ground. I want to know the event that happened in t<0 that initiated this chain of new events i.e.
    pushing of the ground
    Ground pushing me etc.,

    If there is no such event, then does it mean that independent chains of events exist? i.e. events can occur without being a part of a bigger chain of events.
  17. May 10, 2007 #16
    This sounds like it may turn into another "free will" discussion.

    I'm not really sure where this is relevant to physics at all; this looks more like philosophy, or maybe biology.

    I think you may receive a better answer on a different forum section
  18. May 10, 2007 #17
    just a shot in the dark but it seems more to me like the OP is about action-reaction: does that mean I am predetermined? what how do my actions get caused?

    I agree with the DyslexicHobo, seems philosophical rather than physical
  19. May 10, 2007 #18
    Just to give you a hint, here, i'm dealing with Stochastic processes (having some relevance to human beings) that were earlier classified as "free-will" "philosophical" or "unworkable" by physicists and mathematicians. I guess this is kinda advanced for most people. Probably this isn't the place to find answers for quesions like these. Hey....... just gave a try.
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