Question

For the study of any kind of motion, we
never use Newton's first law of motion
directly. Why should it be studied?​

Answer 1

Answer:

It should be studied to have idea of inertia

Answer 2

Answer:

Isaac Newton's First Law of Motion describes the behavior of a massive body at rest or in uniform linear motion, i.e., not accelerating or rotating. The First Law states, "A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force." 

This simply means that things cannot start, stop or change direction all by themselves. It requires some force acting on them from the outside to cause such a change. While this concept seems simple and obvious to us today, in Newton's time it was truly revolutionary. 

Explanation:

may be this helps

but yaa,it is actually well not defined