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



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. You and he speak with the same voice on at least one other topic (besides momentum flow)--he's all about space-time diagrams.
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. The downward momentum flows upward into a book and immediately reverses itself and flows right back against the incoming stream in the opposite direction. 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) I think you need to address how the momentum gets from the earth to, say, the top book without passing upward through the next-to-top book. (If it goes through the next-to-top book, that would suggest that the next-to-top book is passing downward momentum along to the top book which would mean the next-to-top book is exerting the downward force on the top book that we generally think of as the gravitational force of the earth on the top book and that is at odds with gravitational theory.) Does the gravitational field shepherd the momentum through the next-to-top book in such a manner that the momentum in question never belongs to the next-to-top book but flows directly from the gravitational field to the top book?

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
Sent: Monday, October 14, 2013 1:12 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] ? FCI --> momentum flow

On 10/14/2013 08:43 AM, Marty Weiss wrote:
Is diSessa saying there is no force between your butt and the chair?

OK, these are good questions, please keep 'em coming. The questions tell
me there are gaps in the story I'm trying to tell.

Just now I added a section that addresses this:
http://www.av8n.com/physics/force-intro.htm#sec-balanced-forces

There is a stack of books which sits on a scale, which sits on a table, which sits
on the ground.

This is an equilibrium situation. Downward momentum flows in a closed
circuit, upward from the earth to the books, downward through the scale,
downward through the table-leg, and back into the earth.

My students always ask where does it end?

It never ends. The momentum flows around and around, in a closed loop, a
closed circuit, with no net accumulation anywhere. This is characteristic of
equilibrium. For details and diagrams, see
http://www.av8n.com/physics/force-intro.htm#sec-balanced-forces

In my experience, students don't have a problem with this.
They have a pretty good intuition about conservative flow in a loop. If
anybody isn't happy with this, bring in a big round bowl of water and stir up a
rotational flow.
The water is conserved. The water is flowing, but there is no accumulation
anywhere.
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