The first law of inertia deals with the motion of "ISOLATED BODIES".
What are these isolated bodies?
How do we know that a body is free from all forces?
and eventually What do we mean by forces in this context?
This all starts as we consider the forces acting at a distance like gravitation. This creates a confusion in the mere concept of force for me.
As Sir Eddington argues that Newton just postulates the existence of a force whenever we see a body exhibiting a non uniform motion like a parabolic motion (obviously in a frame of reference fixed to Earth). He postulates that there lies a force an external agency there that pressures the body to refrain from its natural motion.
I want to know that is this true.
Is the law of inertia merely a postulate?
If not then how we are going to define forces and so the isolated bodies?
Further Ludwin Lange has defined an inertial frame in his papers. THEN Can anyone explain what was his opinion on the concept of forces?
I want to know what Lange meant by isolated free bodies?
Please mention any online source for this.