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Newton's Third Law, Universe Imploding?

  1. Mar 22, 2008 #1
    If everyone believes that there's an equal and opposite reaction for everything, should the universe go out to a point and stop; or will it end up coming back to its origins?
     
  2. jcsd
  3. Mar 22, 2008 #2

    mjsd

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    are you sure you can study the entire universe using Newton's Laws?
     
  4. Mar 22, 2008 #3
    Newton's 3rd law says that there's an equal and opposite reaction for every force, not for every event.
     
  5. Mar 22, 2008 #4
    I'd say Newton's Laws aren't applicable, depending on the collective velocity of the expanding universe. Remember, Newton's Laws are also only a special case of more complex theories, i.e. Relativity/Quantum Mechanics.
     
  6. Mar 22, 2008 #5

    you are just pick apart his question, and not answering it. assuming he is referring to the force that pushed the universe apart "the big bang," what should the outcome be there is an opposite and equal reaction to that force?

    thanks for actually answering the question:]
     
    Last edited: Mar 22, 2008
  7. Mar 23, 2008 #6
    The reaction force in an explosion is just the explosive force itself. Consider two astronauts in deep space, if A pushes against B then there will be a reaction force from B on A, causing both astronauts ot fly off in opposite directions. The reaction force never at any moment causes the astronauts to move back to each other.
     
  8. Mar 23, 2008 #7
    On the contrary, there appears to be a great deal of evidence supporting the notion of the universe's expansion accelerating, opposed to "coming back to its origins". The third law doesn't really describe this scenerio, it's used in the context of force-reaction force pairs..i.e. You are standing on the ground, exerting a force on the ground. The ground is also exerting a force on you that is equal to the force you're exerting. If it weren't, you would be accelerating up or down (either through the floor or with it!).

    Also--keep in mind; the universe's "origins" is an incredibly sketchy term. The big bang concept is often mis-represented as the universe expanding from the center of space into what we are now, but this isn't accurate since the entirety of the universe WAS the big bang. It was not submerged in some other space, it WAS space (unless you're looking at other models of the universe's evolution like the cyclic model).
     
  9. Mar 23, 2008 #8
    It is nonsense to assume that the a "force pushed apart the universe." Furthermore, just because a force is responsible for inflating a balloon, does that mean that Newton's 3rd law implies that the expansion of the balloon must one day stop or reverse? No.

    The only way to answer ill-founded questions is to point out that they are such. Unfortunately you are propagating the misconceptions by acting as if the original question made sense, by granting the assumption that a force pushed apart the universe (absurd) but resolving the situation by saying that Newton's 3rd law didn't apply to this force.

    My point is the same, the expansion of the universe is not caused by a force, and there is no force to speak of in this situation to which we could apply to Newton's 3rd law.
     
  10. Mar 23, 2008 #9
    The only way to answer ill-founded questions is to point out that they are such. Unfortunately you are propagating the misconceptions by acting as if the original question made sense, by granting the assumption that a force pushed apart the universe (absurd) but resolving the situation by saying that Newton's 3rd law didn't apply to this force.



    but there is still no reason to make someone feel bad for asking, you can corect a person with out being rude about it.
     
  11. Mar 23, 2008 #10

    Dale

    Staff: Mentor

    Either situation is possible by Newton's 3rd law. Newton's 3rd law just ensures that the total momentum is conserved which can happen with an ever-expanding universe or a collapsing universe.

    However, as has been pointed out, it probably isn't best to try and analyze the universe using Newton's laws, nor is it useful to think of the big bang as being like an explosion from a point into an already existing empty space.
     
  12. Mar 24, 2008 #11
    I haven't seen any rudeness in Crossons response. On the contrary, he gave clear and compact answer to the question, unlike the one you credited.
     
  13. Mar 24, 2008 #12
    Agreed, I believe newtons 3rd law was really just a re-statement of the law of conservation of momentum which was already known before him.

    But he just reworded things in terms of forces, which in many respects are easier to play around with. His laws of motion defined force, but they didn't really add anything new to the already established laws of motion.
     
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