Re: [Phys-L] feeler-dealer, third law, et cetera

[John Denker <jsd@av8n.com>]

In the classic novel _A Wizard of Earthsea_ we learn that
a major part of a wizard's job description involves finding
the names for things, and calling things by their true name.

I mention this because on 12/11/2013 03:24 PM, Rauber, Joel wrote:

> I say False to the first question, as there may be more than one
> other object causing the F on the original object, therefore -F could
> be partitioned amongst more than one other objects.

Exactly.  That's the answer I had in mind.

So we have a dilemma:
 a) We want a concept of "force" that upholds the third law, and
 b) we want a concept of "force" that upholds the vector-space axioms.

These are not the same thing.

So let's call each of them by its proper name!

============

We introduce the concept of /simple/ force.  Simple forces occur in 
pairs, and uphold the usual statement of the third law, literally 
and directly.  Alas, the set of all simple forces is not a vector 
space, because it is not closed under addition.

Note that the feeler/dealer concept applies to simple forces only.

Simple forces are an important /subset/ of the set of all forces.

This is discussed in more detail -- with words, diagrams, and 
equations -- at
  http://www.av8n.com/physics/force-intro.htm#sec-simple-compound

As Jeffrey Schnick pointed out, to be 100% technically correct we
need to worry about action-at-a-distance, but I was careful to specify 
the nonrelativistic limit, in which case the simple picture of an
object interacting with other objects is an excellent approximation.
In any case, the momentum-flow viewpoint always gets things right,
with or without the nonrelativistic approximation.