Need of Newton's first law [duplicate]

Question

The second Newton's law of motion is : $\vec{a} = \frac{\vec{F}net}{m}$

I wanted to ask what was the need to define first law when we can easily derive from second law that when $\vec{F}net=0$ then ${\vec{a}=0}$.

Answer 1

I found on French Wikipedia something that answers your question

For a body subjected to a resultant of zero forces, we find Newton's first law, that is to say a uniform rectilinear motion. At first analysis, one may wonder what is the usefulness of the first law since it seems to be a consequence of the second. In reality, in Newton's statement, this is not the case because the first law is not presented as a particular case of the second but as a sufficient condition for the application of the latter.

Indeed, to state the first law is first of all to affirm the existence of the Galilean references. This constitutes an extremely strong postulate which allows, in modern presentations of classical mechanics, to define the Galilean frames which are the only frames in which the second law is valid. In the absence of the first law, the second law is inapplicable since we cannot define its domain of validity. Consequently, the logical order in which the laws are stated is not the result of chance but that of a coherent intellectual construction.

Then, this first law states the principle of isolation of the solid: we consider the external forces which act on it, and we do not take into account what happens internally.