The nature of Newton's laws of motion The first law of inertia deals with the motion of "ISOLATED BODIES".


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*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? 
 A: Let us start from the simplest one: what is force? We can say the force acting on a particle is anything that can change its state of motion. State of motion relative to what? To a inertial reference frame. Then we arise to the main point: What is a inertial reference frame? According to Newton is a system moving with constant velocity relative to the absolute space. There are no logical weaknesses in this Newton's view, but there are ideas which are physically "improper", though. The absolute space have no relation with particles in the Universe and this does not seem to be a reasonable hypothesis. Moreover, as enunciated by Newton, the first law is simply a corollary of the second law.
An alternative view, which is due to Mach and Berkeley states the first law in the following way: 
Given an isolated particle, there is a reference frame, called inertial, relative to which the particle has constant velocity.
As you can see it does not make use use of absolute space and it is a definition of inertial frames. 
In summary you start with an isolated particle (non interacting of far from everything) and then any reference frame with constant velocity relative to this particle will be inertial. Pick on of them. Now you can bring another particle which will interact with the first one and you will see its state of motion changing relative to the chosen frame. What causes this change is called force.
