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Why is momentum conserved?

  1. Jan 17, 2004 #1
    Why is momentum conserved?
  2. jcsd
  3. Jan 18, 2004 #2
    Because of newton's third law.
  4. Jan 18, 2004 #3
    And how do you think the momentum is conserved
    thanks to Newton's 3rd law in a case of lever
    with different distances?

  5. Jan 18, 2004 #4
    I'm not exactly sure what you are asking. Are you talking about the conservation of angular momentum?
  6. Jan 18, 2004 #5
    Re: Momentum

    This really isn't an answer but you might be interested anyway. :smile:

    http://www.emmynoether.com/noeth.htm [Broken]

    This link will take you to a page that discusses the connection between symmetries of physical laws and conservation laws.

    Of course, you have probably already found information comparable to this in your own search.

    The problem is when you answer the question, "why is momentum conserved?" with the answer, "because of the translational symmetry of space.", you have to ask the question, "why does space exhibit translational symmetry?" and on and on...

    I don't think anyone really knows why this is. It is just something that has been repeatedly observed throughout the history of science.

    I thought that was a good attempt at an answer. Of course if you asked, why do forces result in opposite and equal forces, someone might say, because of the conservation of momentum. It's all related.
    Last edited by a moderator: May 1, 2017
  7. Jan 18, 2004 #6
    Re: Momentum

    Nobody knows. It's a law of physics just like the conservation of energy.

    re - Because of newton's third law. - In systems for which Newton's third law holds then the Newton's Third Law can be used to show that mechanical momentum is conserved. However it can't be used to prove that the momentum of an electromagnetic field is conserved. That is a postulate - i.e. the expression for the momentum of radiation can be found my demanding that total momentum be consever.

    This boils down to saying that this is a law of physics and cannot be proven.
  8. Jan 19, 2004 #7
    If nobody knows why how do you know it is true?
  9. Jan 19, 2004 #8
    Experimental observation
  10. Jan 20, 2004 #9
    Have you checked every case?
  11. Jan 20, 2004 #10


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    Nor have we checked that every massive body exerts a gravitational force.

    The nice thing about the universe is that it is pretty consistent. Once you learn the proper parameters to watch it is very predictable.

    Physics does not address why phenomena occur that is the job of philosophers.
  12. Jan 20, 2004 #11
    Actually it was you who used the term "true" not I. I should have clarified but I neglected to.

    Something is said to be a Law of Nature when a relationship between physical quantities is assumed to hold in all possible cases. The dictionary definition is as follows
    That is the reason I stated that Nobody knows.

    Given this assumption of nature one then performs experiments to test that assumoption. If after numerous variations of experiments no counter example can be found one gains confidence in that law. However that never means that the assumption/law is "true" since it is impossible for all possible cases to be tested. This holds for all the laws of nature.

    That is why they are called "Law of Nature" rather than "truths of Nature."

    If you like then just forget the notion "Total momentum is conserved" and rephrase it as you like according to the above reason. Call it the postulate of momentum conservation or whatever.

    However that doesn't mean that there is some uncertainty in the minds of physicits about thins. We're as confident about the certain laws of nature as we are about whether the sun will rise tommorow. neglecting the fact that the Earth might be destroyed in the mean time etc.)
  13. Jan 20, 2004 #12
    Where I come from I expect more than speculation form scholars.

    Whether you are smart enought to understand it or not is debatable but your comments imply that philosophy is superior to physics.
  14. Jan 20, 2004 #13
    Who said anything about speculation? I certainly didn't. Speculation literally means -to review something idly or casually and often inconclusively, to take to be true on the basis of insufficient evidence. You're confusing speculation with a law of nature. They most certainly are not the same thing.

    Suppose I go into the kitchen and fill a pan with water and place the pan on the stove. I then turn the gas on and light it so that there is a flame under the pan. It is not speculation to assert that the pan will get hot or to assert that the water will boil. Even a child knows that to be a fact of life. How can it be proved that this will always happen? It can't be proved. But that doesn't mean that we don't have any idea of what will happen.

    Do you know what a law of physics is or when/how they come came to be?
    That's not a meaningful statement. You're trying to compare apples an oranges. Philosphy and physics are different things but are related to each other. In fact physics used to be refered to as natural philosophy.
  15. Jan 20, 2004 #14
    I thought blind faith was something scientists criticized religion for.

    You are so stupid it is pathetic. You physics people need to learn to think.
  16. Jan 20, 2004 #15
    Nobody is going by blind faith. You simply don't understand the scientific method. I've tried to explain it to you but you seem more interested in insulting me that learning

    By the way - this is a moderated newsgroup. Insults are not tolerated here.
  17. Jan 20, 2004 #16


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    Physicists didn't make up a pretty story about momentum conservation. They observed it to happen over and over for hundreds of years. And every attempt to say, "Hey, I've got a situation where momentum isn't conserved" turned out to be false. Science has nothing it can prove as math does. Everything is always on the table. Not everybody agrees with Karl Popper, but he said science is the collection of propositions that can be falsified. You think you can do better, good luck.
  18. Jan 20, 2004 #17
    You may not have said anything about speculation but the fact that you don't see that you are speculating shows how little you know. Since you can not establish pervasively that momentum will be converved in all cases you are speculating. Speculation is the opposite of certainty. It is making a statement without the support of a valid reason.

    In fact, what you are dong fits perfectly the definition of speculation you gave above.
    This can be established if you understand logic, reason and relations.

    How can you compare forces to momentum? They are different things but related? See how stupid your logic is. Since you need it explained to you here is what I meant.

    Both philosohy and physica are concerned with what exists. Since philosophy understands what is necessary for something to exist it is superior. You physicists don't even know what criteria something must have to exist. You think that everything that exists must be detectable with one of the 5 senses. You have no idea how limited your knowledge and methods are.
  19. Jan 20, 2004 #18
    I didn't realize stating the truth would be considered an insult. You are insulting me with your ridiculous comments. You are going on blind faith because there are two kinds of faith. That which is based on valid reasoning and that which is not based onvalid reasoning. You people do not have any understanding of valid and invalid reasons. Just because you observe it does not necessitate that the observation is valid. In fact, the senses are known to be deceptive.

    The downfall of physics began when sense based observation became more important than logical consistency. If it exists it can be established by a valid reason, if it doesn't exist it can not be established by a valid reason.

    Take your time replying here. I am sure this is all new for you and you are having a hard time understanding it. That is ok though, you are a physicist.
  20. Jan 20, 2004 #19
    I know I can do better and have said how in my posts. What you need to do is study Buddhist philosophy. What you are doing now is not very deep compared with Buddhist philosophy. Until you are exposed to real thinking and knowledge you will not understand how limited your present views really are. I speak from experience. I have studied physics more in depth than most people. I have degress in both physics and math and most of all nothing I have said here has been refuted by anyone. On the other hand, everything which has been presented to me I have dealt with and refuted.

    You criticize religions yet you have countless more blind faith than Buddhist and Hindu scholars.
  21. Jan 20, 2004 #20
    Nobody said that physicists don't have valid reasons to form a postulate. In fact they are only made when the evidence in support of the postualte is overwhelming.
    Yes. They are different things and are related. What's your point?
    All I see so far is that you have a poor understanding of what a law of physics is and how they come to be. You've incorrectly concluded that I was stating or implying the philosphy is superior than physics when in fact I made to such claim and nothing I wrote can be read to imply that. Your claim that I implied that philosophy was superior to physics is meaningless. While you have meant something by it is another thing altogether.
    Nonsense again.
    More nonsense. Apparently you think that the term "observe" as used in the field of physics means to sense with one or more of the human sense - that is incorrect.

    re I didn't realize stating the truth would be considered an insult.

    When you post a message like
    and then imply that you're not insulting someone then you should stop posting and learn more about what is considered flaming.

    Since you're use of logic and philosphy are that of a freshman in highschool I'm not about to bother with you again.

    I highly suggest that you actually pick up a book on the philosophy of science and learn exacly why what you've posted is as wrong as it is.

    When you've changed your mind and decide that you want to discuss physics in a more adult manner, rather than insulting someone because you think your smarter, then try posting again.

    Moderator - I think we can do without tenzin's insults. Please inform him that such behaviour is unacceptable here. Thank you.
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