Question

if newton's 2nd law could prove 1st law why there is a need for 1st law

Answer 1

because netwon 1 law give us the root cause named inertia.i think it

Answer 2

Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia.