Re: Why does inertia exist, what is the atomic explanation?

Fantastic question Aviv!

First, great observation about Sir Isaac. His laws of gravity and motion, 
are observationally derived. That is by studying nature, doing 
experiments, ... he deduced these properties of matter. The law of 
Inertia actually owes its origin to Galileo Galilei (1564-1642). This was 
refined by Sir Isaac Newton (1642-1727). In a nutshell, the law of 
inertia states that objects at rest, will remain at rest if not acted 
upon by an external force (or if the net force is zero - say two equal 
but opposite forces acting at the same time) - and an object in motion 
will likewise continue in motion in a straight line unless acted upon by 
an external force. Newton's 2nd law is a derivative (no pun intended) of 
his first law - that is if a force acts on a body, the body will 
experience a change in velocity and that change in velocity (over the 
time interval - dv/dt, in other words: acceleration) is proportional to 
the force applied and in fact is F = ma where F is the applied force, m 
is the mass (a property of matter) and a is the acceleration.

So inertia is a property of matter, buy why you say? Good question! 
Modern theory states that matter can ultimately be reduced to subatomic 
particles - i.e. a table top, if you would look closely enough (really 
closely) would be seen as an array of atoms interacting with each other 
via the electromagnetic force, and is you look closer still, those atoms 
are composed of a small dense nucleus and a cloud of electrons. That 
nucleus is then composed of protons and neutrons and they in turn are 
composed of quarks - requiring 3 quarks to make an individual proton or 
neutron. This is as far as the Standard Model, our level of understanding 
in quantum physics takes us. Why then do particles have mass and inertia? 
Good question! 

Quantum theory offers up the Higg's field (mid 1980s), which predicted a 
Higg's particle. And the search has been on since then, but no conclusive 
evidence for the Higg's particle, and hence the Higg's field. Another, 
even more esoteric field (string theory) is able to endow matter with 
this property of mass - but at the expense of 11 dimensional space-time. 
The area of study is highly mathematical, but I'd point you to Brian 
Greene's book "The Elegant Universe" if you want to gain a basic 
understanding of string theory (this is not a text book, so you don't 
have to worry about being buried in math should you read it). An even 
more interesting (an controversial) arguement was made a few years ago 
(mid 1990s) that Newton's first law was derivable from Maxwell's 
equations of electromagnetic fields (in the case of zero-point field) - 
making it a form of the electromagnetic force and not a basic property of 
matter as assumed. This is obviously controversial - most hold to the 
idea that inertia is a basic property of matter. Still, stranger things 
have been found to be true.

Summarizing - The law of inertia is an observation. That matter is indued 
somehow with this property is a postulate (not derivable). Some theories 
do attempt to offer up explanations of the origin of this property - but 
much, much work still needs to be done before success can be declared. 
And 
if you keep asking good questions, you may be the one to find that 
answer.  For more info., check out these links:

 
http://www.calphysics.org/articles/zpf_jpc98.pdf

 
http://www.calphysics.org/inertia.html

 
http://www.superstringtheory.com/

 
 http://www.lassp.cornell.edu/GraduateAdmissions/greene/greene.html