Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is Newton's third law applicable everywhere?

  1. Nov 7, 2014 #1
    1. If Big Bang really happened, then what are the action and reaction forces acting?
    2. A quantity like Entropy is always increasing. If average temperature is made constant then universe is getting heated continuously..then what is that being cooled?
    3. Total of action and reaction force =0, if that's true then why universe getting expanded day by day?
    4. Some say that action and reaction force both can not be equal at same time instant..is that true?
    5. In EM field Newton's 3rd law fails totally! So, in our universe, this is a dumb!
    6. Adding a link....Explain me why its happening?
    http://www.animations.physics.unsw.edu.au/jw/Newton.htm
    7. What are action reaction forces in black hole?
     
    Last edited: Nov 7, 2014
  2. jcsd
  3. Nov 8, 2014 #2

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    It seems you have some very basic mis-understandings of Newton's third law. It applies to forces only. The word "action" vs. "reaction" should only be looked at in the context of forces, not abstract phenomena such as the big bang.

    I would suggest studying a bit of what Newton's third law actually entails, because once you know what it really means, you will understand why some of your questions don't make sense. http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law

    In fact, the link you gave is a pretty good introduction of Newton's laws, and you should take a closer look at it. :)
     
    Last edited: Nov 8, 2014
  4. Nov 8, 2014 #3

    Drakkith

    User Avatar

    Staff: Mentor

    1. Matterwave answered this already.
    2. I don't know what you mean. The average temperature of the universe is falling, not holding steady. The very early universe was extremely hot, milllions upon millions of degrees. Expansion has drastically cooled down the average temperature of the universe over the last 13.7 billion years.
    3. Action reaction forces don't apply to the expansion of space.
    4. As far as I know there is always a reaction force equal in magnitude to the action force.
    5. Why do you think Newton's Third Law doesn't work in electromagnetism?
    6. I'm sorry but you need to explain exactly what you'd like to know in your post, not post a link asking someone to explain it.
    7. Gravity. An object approaching a black hole is pulled by the black hole and pulls on the black hole itself.
     
  5. Nov 8, 2014 #4

    OldEngr63

    User Avatar
    Gold Member

    The question about Newton's Third Law in relation to forces derived from electric currents is not entirely settle yet (yes, I know that is heresy, but there are folks who insist that the Third Law should apply, and they have some very interesting arguments). The original work by Ampere described a force that does obey the Third Law, and Maxwell himself regarded this as some of the greatest work ever done, never to be superseded. It was only about the turn of the 20th century that there was the sudden shift in thinking to the Lorentz force law, and that was based on the flow of electrons in a vacuum, not the conduction of currents in a wire.

    For any who might be interested, I recommend Inductance and Force Calculations in Electrical Circuits, by Bueno and Assis. Prof. Assis has made quite a name for himself challenging the currently accepted wisdom in electromagnetic theory.
     
  6. Nov 8, 2014 #5
    Well! guys I've misunderstood the laws, but something I need to know, is mass always a constant or it is a variable quantity?
    I had found an article in the web questioning these things.....reopening that page is now blocked by google says contain malicious software! The article gave some points that break newton's laws of motion. Something I remember was like this...
    F=d/dt(P)=d/dt(mv)=mdv/dt+vdm/dt=ma+vdm/dt;
    The author concluded that, we can not use mass as constant and its a variable quantity in the universe...
     
  7. Nov 8, 2014 #6

    atyy

    User Avatar
    Science Advisor

    It is correct that mass can be variable in Newton's second law. http://en.wikipedia.org/wiki/Variable-mass_system

    However, it is wrong to conclude from that that mass in the universe is not constant. In Newtonian mechanics, mass is conserved when we consider the whole Newtonian universe. However, Newtonian mechanics is not a good model of our universe for particles moving at high speeds. In such cases, special relativity is a more accurate theory, and rest mass is not conserved, but total mass-energy is conserved. http://www.pma.caltech.edu/Courses/ph136/yr2011/1102.2.K.pdf

    Incidentally, within Newtonian mechanics, Newton's third law is equivalent to the conservation of momentum. Although the third law itself does not generalize straightforwardly to special relativity, the conservation of momentum does generalize to special relativity. So it is true that there is no simple analog of Newton's third law in electromagnetism, because electromagnetism is a special relativistic theory. However, the conservation of momentum does hold in electromagnetism.

    http://farside.ph.utexas.edu/teaching/em/lectures/node28.html
    http://farside.ph.utexas.edu/teaching/em/lectures/node91.html
     
    Last edited: Nov 8, 2014
  8. Nov 10, 2014 #7
    I'd like to expand on to Drakkith's reply a little, and explain a little bit why reaction forces don't apply to the expansion of space;

    Newton's third law doesn't apply to the expansion of space for two reasons;
    1. Newtonian mechanics are superseded by relativistic mechanics. Although Newton's laws are extremely precise in everyday settings, when energies get exceedingly large and velocities get near the speed of light, you must use relativistic mechanics (special or general) instead.
    2. Expanding space doesn't really "push" on anything. To get an idea of how space expands, think of a balloon, barely inflated. The surface of the balloon represents our universe. Different points will all move away from eachother as the balloon expands. Note, that the expansion of the balloon doesn't actually "push" any of the galaxies in any direction; it uniformly increases the distances in-between them. So there's no "force" "pushing" them in any direction. Space itself is just growing larger.

    466321a-f1.2.jpg
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is Newton's third law applicable everywhere?
  1. Kepler's Third Law (Replies: 13)

  2. Kepler's third Law (Replies: 2)

Loading...