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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.