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This question already has an answer here:

the first and second laws of newton are both similar, so why do we need the first law if it is the same as the second?

what is the big difference between these?

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marked as duplicate by ACuriousMind, Carl Witthoft, John Duffield, user36790, Gert Nov 30 '15 at 15:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Possible duplicate of Why is Newton's first law necessary? $\endgroup$ – ACuriousMind Nov 30 '15 at 12:04
  • $\begingroup$ @ACuriousMind : thank you for your response , I saw that link before I post this question , but I need more details $\endgroup$ – Parsa mhn Nov 30 '15 at 12:08
  • $\begingroup$ The 1st law gives an experimental definition/test/verification of what a straight line is in this 3D geometry. $\endgroup$ – hyportnex Nov 30 '15 at 12:48
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Although this has been answered many times already, anywhere on this site, the following holds:

First law (existence of inertial reference frames)

There exist in the universe some very particular reference frames where a point particle not subject to external forces moves in a straight line, i. e. $\dot{\textbf{p}}=0$.


Second law (equation of motion in the above defined)

In the very particular reference frames defined by the first law, whenever external forces act on the point particle, the equation of motion is of the form $\dot{\textbf{p}}= \textbf{F}(\textbf{r},\textbf{p})$.

I do not see how the two are similar. Also check this other answer of mine.

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