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Why are Newton's 3rd law forces equal?

  1. Jun 6, 2015 #1
    I am not a physics expert or a math expert, but I do not see this question answered by searching with Google. The opposite forces described by Newton's third law, why are they equal? Why does not the pushed on object push back 1/2 as much, or 2.3% as much, or some other value than exactly the same as the force applied to it. What actually causes the forces to be equal?
     
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  3. Jun 6, 2015 #2

    Orodruin

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    If you look at Newton's laws as fundamental, they are equal because the third law says so. This is how science works, you give a hypothesis which gives testable predictions and then you test it. The third law has been very well tested and so we conclude that it is a good description of how the world works.

    If you want to consider the conservation of momentum more fundamental, then the third law is a consequence of momentum conservation (in the other viewpoint, it is the other way around).
     
  4. Jun 6, 2015 #3

    SteamKing

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    Why is F = ma? Why not F = 2 ma or F = (1/2) ma?

    In my student days, I heard an anecdotal story about a final physics exam which was given. When the students opened the exam paper, there was a single question on it.

    Supposedly, as the story goes, the question stated, "F = (1/2) ma. Re-derive physics."
     
  5. Jun 6, 2015 #4
    Answering that it works because it works does not seem to address the why of my question. There is a why behind all things and I do not see that answered.
     
  6. Jun 6, 2015 #5

    SteamKing

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    Unfortunately, physics is not in the business of answering the "Why?" question.

    Physics, as Orodruin stated, tests experimentally various hypotheses which are offered about how the universe works. Answering the "Why?" question often leads one from the scientific to the philosophical.

    Newton's Second and Third Laws of Motion both offer testable hypotheses about motion. If experiments show that these hypotheses did not accurately describe how bodies move, then these laws would be replaced by different ones, and the process starts over.
     
  7. Jun 6, 2015 #6
    At this point it sounds like the answer is in fact "I don't know."
     
  8. Jun 6, 2015 #7

    ShayanJ

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    SteamKing is saying that physics only cares about "how universe works?" not "why does it work that way?", so any "why?" question is out of physics. If you want to know why universe works the way it does, you shouldn't study physics but philosophy. But if you want to stay in physics, you should get used to it. That's part of understanding physics!
    So the proper way of saying it is, physics doesn't care about "why?" questions, so for a physicist its a choice to care about them or not. But if s\he does care, then s\he's thinking about something other than physics.
     
  9. Jun 6, 2015 #8
    Fair enough, HOW is it that the opposite forces are equal, and not another ratio-relationship? I think you are misreading my usage of why, or perhaps I should use the word how, that really seems like a semantic issue. Regardless, how is it an object does not push back with half or a third the amount of force, instead of equal force?
     
  10. Jun 6, 2015 #9
    Or, what is it that is occurring that is responsible for this equal relationship?
     
  11. Jun 6, 2015 #10

    ShayanJ

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    Your problem is that you think there is an action and then there is a reaction and so the situation seems asymmetric to you. But its not like that. Maybe its just bad wording. The situation is completely symmetric between the two objects. Otherwise how can you say which force is action and which is reaction?!

    Let me explain how this is different from answering a "why?" question. In this explanation, I observe the nature, the way objects interact. I realize the symmetry and make my laws somehow that they respect this symmetry. So its just that I'm modifying my laws so that they work, then I say they're correct because they work.
     
  12. Jun 6, 2015 #11
    What is the cause for things being completely symmetric between the two objects instead of an action and a reaction? This is still the crux of my question.
     
  13. Jun 6, 2015 #12

    ShayanJ

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    Why do you think things should be asymmetric? Do you have a definition that tells you which force is the action and which is the reaction?

    Note that I'm not telling you're wrong. If you can define action and reaction somehow that in every problem you can identify them, and then propose laws with that asymmetry that agree with experiments, then you have proposed an alternative theory to Newton's, just because it works!
     
  14. Jun 6, 2015 #13
    I certainly don't have the expertise or talent to outdo Newton. But it is not that I think that things should be asymmetric, I am trying to understand how it is that they are not. What is the nature of this relationship that makes it as such?
     
  15. Jun 6, 2015 #14

    ShayanJ

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    Its simple. Didn't you notice we always say "two objects"? How can the situation be asymmetric? Is it meaningful to you that I ask "what is the nature of this relationship that makes us say two objects?","why shouldn't we say object1 and object2?" ?
    The point is, we're assuming nothing about the two objects. As far as we are concerned now, they're just two objects! There is no difference between them because we didn't assume it! So its really natural that when the description is symmetric, the laws should be symmetric too.
     
  16. Jun 6, 2015 #15

    Nugatory

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    Yep... No one is disputing that.

    "Why?" questions can never be completely answered, as the answer to any will lead to another "Why?" question. In this case, we can say that the a force and its reaction force are equal because otherwise momentum conservation would be violated... but that just invites the next question, "WHY is momentum conserved?". The cycle terminates only because we get to something that we can agree to accept as a given, and that's where we are with Newton's third law.

    We've answered the original question in this thread as well as it can be answered by modern physics, so we're closing it now.
     
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