# Difference between first newton's law and second newton's law ? [duplicate]

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?

## marked as duplicate by ACuriousMind♦, Carl Witthoft, John Duffield, user36790, GertNov 30 '15 at 15:00

• Possible duplicate of Why is Newton's first law necessary? – ACuriousMind Nov 30 '15 at 12:04
• @ACuriousMind : thank you for your response , I saw that link before I post this question , but I need more details – Parsa mhn Nov 30 '15 at 12:08
• The 1st law gives an experimental definition/test/verification of what a straight line is in this 3D geometry. – hyportnex Nov 30 '15 at 12:48

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. $\dot{\textbf{p}}=0$.
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 $\dot{\textbf{p}}= \textbf{F}(\textbf{r},\textbf{p})$.