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

[John Denker <jsd@av8n.com>]

I think this is a nifty discussion.  It is hard to imagine any topic
more suitable for phys-l.



On 12/12/2013 05:23 AM, Philip Keller wrote:

> When I state the third law in class, I begin: "when an object A exerts a
> force on object B..."
>
> This formulation evades the issue of whether the third law applies to the
> sum of the forces.  And I am glad to evade that question

That sounds 100% fine, especially as a place to /begin/.

>  I care about the
> sum of the forces that act ON object A because that sum determines object
> A's acceleration.  I don't care about the sum of the forces exerted BY
> object A because it is likely that they are acting on an assortment of
> objects, each which are experiencing their own collection of forces.

What we care about depends on the situation.
 a) There are situations that involve the third law and not the second
  law, and we care about which dealer goes with which feeler.
 b) There are situations that involve the second law and not the 
  third law, and all we care about is the total force.

> I have often told my students that I am unaware of any exceptions to this
> law and that if there were any exceptions, they would violate the law of
> conservation of momentum. 

Right.

> But now I see from Bruce's post about the
> electron adn proton pair that I have some reading to do.

It's not a big deal.  The fundamental idea of local conservation of 
momentum is unchanged.

There is momentum in the electromagnetic field.  This is usually 
neglected in introductory discussions, but it is definitely there, 
and sooner or later you have to take it into account.  The energy 
flow and the momentum density go like E×B so unless you have an 
E-field /and/ a B-field you can postpone worrying about it.

BTW this is typical of how the fundamental conservation laws are 
used:  If you see something that looks like it might be a violation,
the smart strategy is *not* to assume a violation, but rather to 
hunt around and find where the missing stuff is hiding.  For example, 
this is how the neutrino was discovered.

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

On 12/12/2013 09:18 AM, Jeffrey Schnick wrote:

> I think it's probably better not to increase the amount of jargon
> that students have to learn.

We all agree on that, other things being equal.

On the other hand, given the choice between
 a) verbosity, and
 b) ambiguity, inconsistency, and contradiction

then I vote for verbosity every time.


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

On 12/12/2013 07:07 AM, jbellina wrote:
> Seems to me there is logical error in your analysis.  The claim in
> Newton's third is not about "net force" but about individual
> interactions between objects.  Net force is a mathematical construct
> and thus does not necessarily have physical sense in the way that
> individual ones do.  It seems to me to be a false generalization to
> say that because individual forces follow Newton's third, that all
> constructed forces must also.

It can't be much of an error, since Joe B. is saying the same thing
I am saying, just in different words:
  simple force    <--> individual force
  compound force  <--> constructed force

I care a thousand percent more about ideas than about terminology.
So call it whatever you like:  Simple forces.  Individual forces.
Pairwise forces.  Interaction forces.  The point remains that such
forces are a subset of the space of all forces.

There are some situations where we want to treat every kind of
force on the same footing ... and other situations where we
want to focus on the third law.  It's OK either way, so long 
as you recognize that these are not the same thing.

> N3 applies to pairs of objects interacting with each other.  It does
> not apply to the mathematical artifice we call the net force.

Again we agree on the physical facts.  The point of departure is
that I don't see any need for the pejorative value judgment.  I
don't see why the total force is a "mathematical artifice".  If
you really believed that you would call the second law merely a
"mathematical artifice".

This is the sort of thing that drives students crazy:  We use the
word "force" in a narrow way when dealing with the third law, and
then use it in a much broader way when dealing with the second law.

I assert that all FCI scores in the literature are terrible.  Even 
folks who brag in relative terms about the "success" of their "reforms" 
still get terrible scores in absolute terms.

I further assert that ambiguity, inconsistency, and contradiction
as to the meaning of "force" is a nontrivial part of this.