I'm describing a hypothetical universe. I'm assuming that the first law of newton holds true, that means there exists a inertial reference frame. Now the universe has three bodies.

Second law is also true for this universe.

Now if the third law is somewhat different, like $F1 = - 2F2$.

My question if i change the third law does this lead to any logical inconsistencies with the first two laws?

  • 1
    $\begingroup$ If $F_1 = -F_2$ then $F_2 = ?$. $\endgroup$ Commented Nov 7, 2012 at 20:47
  • $\begingroup$ $F2=-2F3$ :)...i'm just saying if its different from actual third law, does not matter they differ how. $\endgroup$
    – Aftnix
    Commented Nov 7, 2012 at 20:51
  • $\begingroup$ (Ah, I should had written "If $F_1 = -2 F_2$ then $F_2=?$".) But if it is an open-ended question, then it does not fit here very well - You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page. (from physics.stackexchange.com/faq) $\endgroup$ Commented Nov 7, 2012 at 21:59
  • $\begingroup$ I'm trying to understand how newton's laws are logically connected. I guess the QA should allow that. $\endgroup$
    – Aftnix
    Commented Nov 8, 2012 at 10:16
  • $\begingroup$ The point is that your proposition is not even self-consistent. So there, by logics, it contradicts everything. When it comes what should be allowed - "Does X imply Y?" is good, but "What are consequences of X?" or "I want to understand Y" are not well suited for Q&A format. $\endgroup$ Commented Nov 8, 2012 at 11:26

1 Answer 1


Newton's third law, states that 'To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions", which is an assertion of the symmetry of interaction.

In principle, Changing any one of Newton's Laws does not necessarily 'contradict' the other two laws, since they are independent postulates - there is no necessity of logic which links them all.

See: Logical connection of Newton's Third Law to the first two

However, one must be careful when changing a 'fundamental law' as any of Newton's laws, since they are so intimately intertwined. Newton's three laws of motion generally go 'hand-in-hand' and are fundamentally taken as premises in any argument about the interaction of matter in classical mechanics.

For an interesting discussion of the nature and implications of Newton's 3rd law, see: Deriving Newton's Third Law from homogeneity of Space

and at: Violation of Newton's 3rd law and momentum conservation


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