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Gravity and Newton 3rd Law

  1. Sep 4, 2006 #1
    Newton's third law states when two bodies interact they exert equal but opposite forces on each other. Now we apply this concept to gravitational attraction between, say, the Sun and the Earth. Sun exerts a gravitational force on Earth, and thus Earth also exerts a gravitaional force on the Sun. Now say that somehow, we can block the gravity of the Sun. So now since Sun doesn't exert a gravitational force on Earth, Earth shouldn't exert a force on the Sun as well according to Newton's third law. But we didn't block Earth's gravitational force, and thus, technically, Earth should still pull on the Sun!

    How can one resolve this apparent paradox?
     
  2. jcsd
  3. Sep 4, 2006 #2

    Janus

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    If you could block gravity (unlikely), and if you could make this blocking effect one-way(extremely unlikely; It would be like saying that object A can touch object B without object B touching object A), Then it would simply mean that Newton's 3rd law doesn't hold.
     
  4. Sep 4, 2006 #3
    My understanding is that there is no gravitational force in the abscence of another mass to interact with. F = Gm1m2/r^2 (Unless there are two masses, then this force is 0). In other words, gravitational force is not a property a mass exhibits constantly, but rather when it is interacting with another mass.

    Now assuming you were to be able to block the gravitational force of the Sun, my guess is that you would indirectly be blocking the gravitational force of the Earth as well so the law would still hold.
     
  5. Sep 4, 2006 #4

    Danger

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    I'm not sure where that came from. Gravity is an intrinsic component of mass. It doesn't matter whether or not there's anything for it to interact with, except that it's the only way we can detect it.
     
  6. Sep 4, 2006 #5
    What you should do, is use Newton's 3rd law to show that you can't block the pull from one mass without blocking the pull from the other (ie separating them to infinity). You can't assume that you can block the pull from one mass only, and say that Newton's 3rd law still holds. You could say that if Newton's 3rd law wasn't true, then you could block the pull from one without blocking the pull from the other. But that wouldn't accurately describe reality. :wink:
     
  7. Sep 4, 2006 #6

    rbj

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    there is no paradox for a hypothetical situation that is impossible. why waste your time on it? i can also dream up a pile of other paradoxes based on fantasy. here's one:

    If Gandalf can summon giant eagles to fight the Nasgool in an air battle at the end, why couldn't he summon them earlier when needed in battle?
     
  8. Sep 4, 2006 #7
    That's a sweet move.:biggrin:
     
  9. Sep 4, 2006 #8
    It would be impossible to selectively block gravity. You cannot selectively block EM radiation so why is this different
     
  10. Sep 6, 2006 #9
    I think this question theoreticaly makes sence.
    Here is what I think would happen. As Sun's gravitational field is blocked, the Earth's would act on Sun. And because Earth acts with force on Sun, the Sun must act on earth too with equal but opposite force. The only difference would be the magnitude of force. It would be Gm/r, where m is Earths mass.
    Thus - no paradox
     
  11. Sep 6, 2006 #10

    rbj

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    none of the above makes any sense whatsoever. not sure it's not a troll.

    "Gm/r" is not a force. nor is it a force per unit mass (which is what you might be implying). it is energy per unit mass (so i think it would have the same dimension as c2). but i have no idea where this 1/r form comes from in the context of this discussion.
     
  12. Sep 6, 2006 #11

    Danger

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    xAxis, either you're a troll as rbj suggested, or you have no concept of gravity. Either way, please keep in mind that this is a serious science site intended to help people with their studies or projects in professional life. There's no room for baseless speculation. I speculate a lot, because of lacking an education, but never unless I have some basis for thinking that my speculation is valid. And at that, I always defer to those more knowledgeable. Any understanding of either Newtonian or Einsteinian gravitational theory should prevent someone from making the assumtion that you did in your post.
     
  13. Sep 7, 2006 #12
    Newton's 3rd law will not be changed over time, it might be slightly modified like his law of gravity for all we know but its going to be generally true, for me i don;t believe that it will ever be overturned or modified.
     
  14. Sep 11, 2006 #13
    Yes, I've probably gone too far with my conclusions. I somehow wanted to point out that the third Newton's law is at least as universal as gravitation law.
    Now let's consider another example. Let's put a boy and a girl somewhere in free space so that they are meter appart facing each other and holding each other hands. If he pulls her with constant forse of say 10N, and she pulls him with force of 5N, what will be the actual force with which he is pulling her?
     
  15. Sep 11, 2006 #14

    rcgldr

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    This isn't possible. They will always pull with the same force. Imagine that they both are holding on to the ends of a rope. Assuming that the rope isn't accelerating (in the direction of the rope), then the tension in the rope will always be the same at both ends. If one of them initiates a 10N force in the rope, then once things stabilize (after the rope stops stretching), there will be 10N of tension in the rope, and at both ends of the rope. If they're not attached to anything else (other than the rope), then they are accelerating towards each other.

    For a simpler example, connect two fishing weight scales (these measure tension and indicate the tension in order to weigh an object, usually a poor dead fish), in series. Have two people pull on each end of one of the scales. Can the two people generate forces so that the fishing scales show a different amount of tension (weight reading)?

    Regarding your question about gravity from only one of two objects, it doesn't matter. Even if you ignore the gravitational field generated by one of a pair of bodies (which is quite often done in the case of relatively small objects interacting with much larger objects, say a bowling ball or a space station, and the earth), the gravitational field generated by the remaining object (for example the earth) still results in an attractive force between the two bodies (for example the earth and bowling ball). In the case of a bowling ball and the earth, almost all of the gravitational force between the two objects is due to the mass of the earth. The earth is pulled towards the bowling ball just as hard as the bowling ball is pulled towards the earth, because of the gravitational effects due to the mass of the earth. The increase in this pulling force due to the mass bowling ball is insignifcant.
     
    Last edited: Sep 11, 2006
  16. Sep 11, 2006 #15

    Where do I start.


    First off: A general rule in theoretical physics--fields are made up. Completely fabricated. They are a mathematical construct with no reality--you cannot measure fields (well, with one exception that I am aware of, but its not gravity). Fields are really just a bookeeping tool to simplify the descriptions of interactions--which are real.

    So you cannot block the sun's gravitational field--it doesn't exist. Further, in General Relativity, gravity isn't even an interaction. So there isn't even an interaction to block. The situation suggested simply makes no sense.
     
  17. Sep 12, 2006 #16
    quote Jeff Reid:

    "Even if you ignore the gravitational field generated by one of a pair of bodies (which is quite often done in the case of relatively small objects interacting with much larger objects, say a bowling ball or a space station, and the earth), the gravitational field generated by the remaining object (for example the earth) still results in an attractive force between the two bodies (for example the earth and bowling ball). In the case of a bowling ball and the earth, almost all of the gravitational force between the two objects is due to the mass of the earth. The earth is pulled towards the bowling ball just as hard as the bowling ball is pulled towards the earth, because of the gravitational effects due to the mass of the earth. The increase in this pulling force due to the mass bowling ball is insignifcant."

    yes, that's exactly what I ment in my first post. I even wanted to take that same example, with Earth and pebble, but was scared out of it by rbj's and Danger's reply :):smile:
    As for general Relativity, gravitation is not even a force, and so I think there is no action and reaction.
    As for my second example, here is how I calculate. The boy pulls on girl with the force of 10N. By reaction he is pulled by 10N force, plus the girls pull of 5N acting also on him, which makes 15N. The same goes for the girl.
    So they are both acted on by the same force of 15N
     
  18. Sep 12, 2006 #17
    No, opposite forces of 15 N.

    Newton's third law is very poorly stated in that form, IMO. Much better the restate it as 'All internal forces in a system sum to zero', which is to say that the interactions of the elements of a system can change their positions relative to each other, but not change the position of the system's center of mass (relative to some external observer).
     
  19. Sep 17, 2006 #18
    Since we are talking about newton's 3rd law, I have got a question on it too. When I push a concrete wall(it is connected to nothing but the floor-no ceiling or neighbor walls) Is the force I am applying to it rebounding to me thus stating newton's 3rd law? But if the force completely rebounds(which it does) where is the damage to the wall?(considering that if I push the wall forever, it will break sometime(just assume I won't break) Because if the force does damages the wall, it would be wasted there and wouldn't completely rebound, unless, when the force damages the wall, the energy released by the damage is equal to the energy required for it to damage. Which is why the rebound energy is equal. Am I right?
     
  20. Sep 17, 2006 #19
    You can't say force is rebounding. It's just there as long as you are pushing. Imagine you are pushing a car. Do you feel heaviness while doing that? Of course you do, and that's the force with which the car pushes you. The stronger you push, the hevier the car will push on you. So the reaction force is not rebounding, it is just there at the same moment as you start pushing.
    The same with the wall. As you push against the wall, the wall pushes against you. It won't break because the strong intermolecular forces resist. When you break the wall, what happend? Some parts of the wall couldn't resist the push as it was strong enough to break the intermolecular forces. Let's say that you manage to break the part of the wall which is exactly one brick. At the moment of breaking, your push was probably the strongest. After that, that brick leaves the wall as it is not intermoleculary bound to it any more, and now you are pushing only the brick, which will quickly go forward. You will unwillingly reduce your pushing force, but at any time the force you are pushing, and the force you are pushed are equal but opposite.
     
  21. Sep 18, 2006 #20
    Lets say that I am a bug and that my force against wall isn't enough to break it, but since I am keep pushing on it till infinity, wouldn't wall run out of energy to equal action to its reaction at some time? If it would, would that be same as breaking the wall by weaking its bonds or would it something else, like wall has used up all of its energy?

    Also, the force I am applying, where does that force go? Does it work like this?

    I apply a force on wall, that force weakens some bond, the energy released by the weakness is the reaction force.
     
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