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B All actions are reactions

  1. Apr 9, 2016 #1
    My grandson asked me this question:
    If in fact,as Newton said,all actions are accompanied by an equal and opposite reaction, then there can be no single actions in the universe;all actions must therefor be part of a reaction between two opposite and equal actions,which are in fact also reactions.
    Any thoughts on this are welcome.
  2. jcsd
  3. Apr 9, 2016 #2


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    Staff: Mentor

    Ok....so, what does that do for us? Is that physics or philosophy?
  4. Apr 9, 2016 #3


    Staff: Mentor

    Yes, it is well known that the labels "action" and "reaction" are completely arbitrary and can be equally arbitrarily swapped without any change in the physics. Most experts just identify a pair of forces as a "third law pair" and never even bother to identify one or the other as the "reaction".

    If you search you should find many threads where this is discussed already.
  5. Apr 9, 2016 #4
    Just remember that Einstein showed us that there's a finite speed of causality...there's no such thing as instantaneous action at a distance. Push the Sun and it'll take a while for the Earth to notice. Also, if I remember correctly, Newton's action/reaction force pairings are violated in certain cases involving electromagnetism.
  6. Apr 9, 2016 #5
    Please give examples of an electromagnetic violation of action/reaction.
    Last edited by a moderator: Apr 9, 2016
  7. Apr 9, 2016 #6
    "Please give examples of an electromagnetic violation of action/reaction."

    As for electromagnetism, imagine two charged particles moving at one another. The electric force will be equal but opposite, but the magnetic force won't be in opposite directions. This seemingly violates conservation of momentum (which is pretty much what Newton's third law is all about), but it's salvaged by allowing the electromagnetic field itself to possess momentum.

    Edit: I shouldn't have said moving directly at one another, the magnetic force would be zero in that case. A better example would be two positively charged particles, one moving on the x-axis one moving on the y-axis, both moving toward the origin. The electric force will be equal but opposite, the magnetic forces will be in the positive y and positive x directions.
    Last edited by a moderator: Apr 9, 2016
  8. Apr 9, 2016 #7


    Staff: Mentor

    Closed pending moderation.

    Edit: the thread is reopened. Several posts have been edited or deleted to remove off topic content. Discussions about entanglement belong in the Quantum Mechanics forum and discussions about the big bang belong in the Cosmology forum. Neither belong in a thread on Newton's third law.
    Last edited: Apr 9, 2016
  9. Apr 9, 2016 #8

    Imagine the two particles at rest.What action puts them in motion?
  10. Apr 10, 2016 #9


    Staff: Mentor

    This is precisely why this terminology is discarded. A gravitational force could put them in motion, but there is no reason to assign one or the other as the "action". Similarly with other forces.
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