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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?

Aftnix
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    If $F_1 = -F_2$ then $F_2 = ?$. – Piotr Migdal Nov 07 '12 at 20:47
  • $F2=-2F3$ :)...i'm just saying if its different from actual third law, does not matter they differ how. – Aftnix Nov 07 '12 at 20:51
  • (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 http://physics.stackexchange.com/faq) – Piotr Migdal Nov 07 '12 at 21:59
  • I'm trying to understand how newton's laws are logically connected. I guess the QA should allow that. – Aftnix Nov 08 '12 at 10:16
  • 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. – Piotr Migdal Nov 08 '12 at 11:26
  • Thats exactly my point, i know my proposition should not be consistent, I'm trying to find out "how its inconsistent". I thought by denying third law, and withholding first two will lead to a trivial inconsistency. The question may be moronic, but i'm simply looking for an answer. "how it contradicts logically possibly the first law". I'm allowed to think of a crazy universe and then say well this crazy universe can't exist because its not even logically consistent. – Aftnix Nov 08 '12 at 11:35
  • More on un-equal action-reaction force pairs: http://physics.stackexchange.com/q/51523/2451 – Qmechanic Jan 21 '13 at 09:49

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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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