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

[John Denker <jsd@av8n.com>]

I have inadvertently made this discussion more confusing
than it needed to be.  This is because I violated one of my 
own rules, namely:  Do not call attention to misconceptions
any more than necessary.

At one point I even managed to confuse myself.

The smart thing is to focus on forces in all generality.

     In contrast, it is a misconception to focus on "forces on 
     objects".  Some forces are simple feeler-dealer forces,
     whereas most forces are not.  For example, they might be 
     formed by adding two or more feeler-dealer forces.

Force is a vector.  The axioms that define what we mean by
"vector" require that the sum of two forces is itself a force.

On 12/12/2013 11:37 AM, Bruce Sherwood wrote:
> Here's an alternative view of Newton's 3rd law, which is the approach Ruth
> and I have taken. Don't claim that Newton's 3rd law is general. Rather,
> show by the symmetry of m_1*m_2 and q_1*q_2 that the gravitational and
> electric force laws show the property of "reciprocity", that F is paired
> with -F. Mention that later we'll encounter magnetic forces that do not
> have this property. The reciprocity of electric forces provides a kind of
> microscopic underpinning for the forces between the big truck and the small
> car having the same magnitude, because the interatomic forces between
> bumpers must have the same magnitude.

That would be a great idea, except that the third law *IS* general!

The third law -- when properly formulated -- is tantamount to 
conservation of momentum.  Sure, there are eleventy million ways
of mis-stating the law so as to make it less-then-general, but so
what?  It is axiomatic that no matter what you are doing, you can
always do it wrong.

I asked a true/false question about a deceptive, mis-stated version 
of the third law, a version that focused far too much attention on 
forces on objects.  We need to move on from this, the sooner the 
better, and return the focus to /forces in general/ where it belongs.

> When discussing magnetic force, have the students calculate the forces in
> the vx-vy case to see the lack of "reciprocity" and draw their attention to
> the primacy of momentum conservation,

If you believe that momentum is conserved, define force as dp/dt and
-- shazzam! -- you've got a nice, general, robust version of the third
law.

Is there a problem with this?  Maybe there's something I'm not seeing.

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

One source of confusion that I sometimes see is that people who accept
the idea that a field can exert a force on a particle are slow to
accept the idea that a particle can exert a force on a field.

At this point it may help to remember that according to the modern
(post-1924) understanding of physics, /everything/ is a field anyway.
The photon is an excitation of a certain bosonic field, just as the
electron is an excitation of a certain fermionic field.  They are
more similar than they are different.

One wonders why people get so hung up on this.  I have some theories,
but I'll keep them to myself for now.

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

I am surprised to get pushback on my diagram
  http://www.av8n.com/physics/force-intro.htm#fig-h-d-cation
that uses electrostatic forces.

By way of analogy, note that in introductory physics it is absolutely
routine to use one weight to lift another weight with the help of
massless strings and frictionless pulleys.  We speak of one weight
pulling on the other weight, and we abstract away the string that
transmits the force.

The same sort of approximation and abstraction is involved in my
figure ... except that mine is a muuuuuch better approximation.
The electrostatic field does a really good job of transmitting 
the force from one place to another.