Re: [Phys-L] ? FCI --> momentum flow

[John Denker <jsd@av8n.com>]

On 10/18/2013 10:24 AM, Jeffrey Schnick wrote:
> John, 1. Have you read Tom Moore's Six Ideas That Shaped physics
> book?  He uses the momentum flow approach and according to his web
> site his students score 90% on the FCI. 

Yes, I've seen that.  I actually bought the book, although it
seems to have walked off.

>  You and he speak with the
> same voice on at least one other topic (besides momentum flow)--he's
> all about space-time diagrams. 

That volume seems to have walked off, too.

> 2. In your diagram for the stack of
> books on the scale on the table on the ground, you have the books
> conveniently off to one side to facilitate the depiction of the
> momentum flow loop.  I think the momentum flow is more of a to and
> fro than a loop. 

Hmmm.  I diagrammed it the way I think of it.  

I cantilevered the books to make the diagram easier to draw,
but I think of it as a circuit, even in cases where the flow
retraces its steps such that the circuit is not so clearly a
loop.  We're talking about this diagram:
  http://www.av8n.com/physics/force-intro.htm#fig-books-on-table

It seems to me that direct "to and fro" cannot be the general
case.  For one thing, the gravitational interaction is strictly
a central force, acting along a straight line from source to
destination ... whereas the mechanical force carries momentum
along a devious path through the table leg.

I don't show it in the diagram, but there are /shear/ forces
involved, whereby there is a sideways flow of downward momentum.
Compare the explicit shear shown here:
  http://www.av8n.com/physics/force-intro.htm#fig-block-sliding

  I'm guessing now, but let me guess that your intuition is
  telling you something about /angular/ momentum.  If a force
  "here" is balanced by a force "there", we get a torque, and
  a flow of angular momentum.  That's a valid and commendable
  intuition, but I have intentionally decided to not discuss
  that in this figure.  The z-component of momentum is separately
  conserved, and I am within my rights to analyze that by itself.
  Yes, angular momentum is conserved also, but that issue can be
  postponed.  There is a pedagogical reason for consolidating the 
  idea of momentum flow using a simple example before moving on
  to other ideas. The books-on-table example doesn't solve all 
  the world's problems, but it is not wrong.

>  The downward momentum flows upward into a book and
> immediately reverses itself and flows right back against the incoming
> stream in the opposite direction. 

I'm still not convinced.

>  Given that momentum is locally
> conserved (you don't mention this on the web page until you start
> talking about angular momentum--but you do mention it) 

We agree that the document is seriously disorganized.


> Does the gravitational field shepherd the momentum through
> the next-to-top book 

I may not be understanding the question properly, but I think
the answer is no.  The downward flow of downward momentum is
purely mechanical.  It is carried by the chemical bonds in the
materials, not "shepherded" by gravity in any way.

If that isn't the desired answer, please re-ask and/or clarify 
the question.