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The Law of Action-Reaction consequence of electron repulsion?

  1. Dec 4, 2011 #1
    Or is there something else causing the action-reaction pair of forces other than the same charge of electrons in colliding objects?
  2. jcsd
  3. Dec 5, 2011 #2
    Nobody knows or is a stupid question?
  4. Dec 5, 2011 #3
    I suspect no one knows what you are asking.

    The electromagnetic force is described here:

    No one knows why any of the forces we observe exist, nor their "cause".

    For example, no one knows why the electron has a any charge, nor why it's magnitude is that described in the above article, nor why there are positive and negative charges.
    But we do observe those and we know to a rather detailed degree how they interact.
    All that we do know is incorporated in the Standard Model of Particle Physics.
  5. Dec 5, 2011 #4
    Isn't it because of U(1) gauge symmetry of the QED Lagrangian? Someone please correct me if I'm wrong
  6. Dec 5, 2011 #5
    Well, my question is not asking about anything that fundamental like why an electron has a charge or anything like that.

    I just want to understand the Newton's third law better. My feeling is that the action-reaction pair of forces are not some fundamental forces described by the Standard Model but just a consequence of more fundamental forces like electromagnetic repulsion that we now know of but Newton didn't know of them at the time he postulated the 3rd law.
    Is the question more understandable now?
  7. Dec 6, 2011 #6
    Electromagnetic repulsion cannot explain why gravity has this property.
  8. Dec 6, 2011 #7
    I'm confused. What does this have to do with gravity?
    I always though the 3rd law was about collisions like billiard balls kicking into each other and similar situations where gravity can be safely neglected.
  9. Dec 6, 2011 #8


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    action-reaction is a general principle that applies to ALL forces, including gravity.
    The "action-reaction" you are talking about with the two billiard balls is a consequence of the action-reaction intrinsic to the electromagnetic interaction, which is the force responsible for the the bouncing of billiard balls.
    But action-reaction happen for all forces, to see where it happens for gravity, consider a planet orbiting the sun. The sun exerts a force on the planet, the principle of action-reaction says the planet then must exert an equal and opposite force on the sun (which is also obtained using the same law of gravitation, with the direction vector turned around). How can we see the effect of this "reaction" force? Well, the planet doesn't actually orbit the sun, both the planet AND the sun orbit the center of mass of the sun-planet system, so the sun itself is also in an "orbit", only it is much smaller because the sun is so massive (in fact the center of gravity of the planet-sun system lies INSIDE the sun, so the sun orbits around a point inside itself).

    For two orbiting bodies of similar mass, the center of gravity lies roughly in the middle of the two objects. This is the case with, for example, binary stars. Both stars in such a system will orbit an "invisible" point, somewhere in between the two stars. The pull of star A on star B causes the orbital movement of star B. The principle of action-reaction tells us than since there is a pull from A on B, there must be an equal pull from B on A, which causes star A to orbit.

    A final note.
    There are no such things really as "action" forces and "reaction" forces, there is no "action" force that "comes first", and, through the principle of action-reaction, "causes" the "reaction" force. The principle of action-reaction merely states that all forces come in pairs.
    In the case of newton's gravity and Coulomb's law, this can easily be seen by examining the force law. Take gravity:
    \bar{F}=\frac{G m_1 m_2}{r^2}\hat{r}_{12}
    where [itex]\hat{r}_{12}[/itex] is the unit vector pointing from mass 1 to mass 2.
    This gives the force from 2 on 1, but of course we can switch the masses around to get
    \bar{F}=\frac{G m_2 m_1}{r^2}\hat{r}_{21}
    where [itex]\hat{r}_{21}[/itex] is the unit vector pointing from mass 2 to mass 1.
    which gives the force exerted by 1 on 2.
    This pair of forces is obviously equal in magnitude, and opposite in direction, they form an "action-reaction" pair.


    Wow, I've written too much, time to go to class :p

  10. Dec 6, 2011 #9
    Ah, much clearer....
    The context is, I think, mostly historical....akin to what you suggest. Action - reaction sure is NOT a fundamental force...but an observation about dynamics which is a consequence of the forces we define. I'd think of the forces as underlying explanations for such interactions.

    The Third Law is generally taken to apply to bodies (masses) while the fundamental forces usually describe such actions in terms of fields....not always masses. Read here:


    and you can pick out other differences....like fictitous forces...and maybe arive at a better
    description than I have suggested....
  11. Dec 6, 2011 #10
    Back to the OP, which I think was getting at: What really happens when two billiard balls collide?

    Yes, your hunch was correct that most everyday contact forces are electromagnetic in nature. More specifically, the atoms in a door are bound together by electromagnetic forces. When I try to push a door open with my hand, the bound electrons in my hand repel the electrons in the door, but because the electrons are bound to atoms which are bound to each other, the force gets transmitted through the door as a whole. But the electrons in the door also repel the electrons in my hand, so a reaction force is transmitted down my arm. If the atoms in the door are not tightly bound by interatomic electromagnetic forces, then when I go to push on it, just a region of atoms near the contact point accelerate instead of transmitting the force (i.e. I punch a hole in a flimsy door and shards go flying).
  12. Dec 7, 2011 #11
    Thank you, Naty1!
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