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

Third newtonian law most fundamental

  1. Mar 22, 2007 #1
    third law is equal and opposite reaction . . . the first law is inertia of mass.

    From everything I can remember reading, people often talk about the mystery of inertia in bodies . . . well, i finally got around to thinking about this stuff(admittedly), and it seems to me that the first law is a derivable from the third. If you try to pull a set of particles all reacting to your pull(and maybe some initial pushing of contacting a body of particles), then, all those particles will push away(and do some back and forth reactions amongst themselves) . . . so, the first law is explainable by the third.

    I've never heard anybody say this, so, sorry is somebody has!
  2. jcsd
  3. Mar 22, 2007 #2
    Nope, there are three laws.

    Ok, so, really Newton's first law reads:

    If a body is subjected to no net force, it receives no net acceleration.

    The Third law reads:

    For every force of "A" on "B", there is an equal and opposite force of "B" on "A"

    So I don't see how you derive N1L from N3L.

    Last edited: Mar 22, 2007
  4. Mar 22, 2007 #3
    If you pull at a set of particles, why would they pull away from you unless there was another force acting on them?
  5. Mar 22, 2007 #4
    In classical electrodynamics we have N1L is true, and N3L is false. This doesn't, however, contradict what you are saying.
  6. Mar 22, 2007 #5

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Note that Newton's First law is not true for all observers. It is only true for an observed fixed in some inertial frame. Think of the first law as defining the concept of an inertial frame.

    The first law only vaguely mentions forces. The second law expounds on this concept, describing what an observer tied to an inertial frame sees when a force is appied to some object. The second law defines force in terms of behavior in an inertial frame. The third law further expands on the concept of force, saying that forces come in equal and opposite pairs.

    One cannot start with the third law as the most fundamental because it depends on the concept of a force", defined in the second law. Similarly, the second law depends on the concept of an inertial frame, defined in the first law. Newton was no dummy. He established these laws in the order he did on intentionally.
  7. Mar 22, 2007 #6
    Also worth noting, is that the first law defining an inertial frame requires that you know how forces act in an inertial frame, which in turn requires the second and third laws. None of Newton's laws are independent, but at the same time they're very vague in the sense of "what exactly is a force?". However, you answer that question by knowing what's acting on what and how.
  8. Mar 22, 2007 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Not at all. It is as precise as "points, lines and planes" are in Hilbert's axiomatization of geometry.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook