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

[Moses Fayngold <moshfarlan@yahoo.com>]

Marty Weiss wrote:
To Phys-L@Phys-L.orgMe
Oct 14 at 11:43 AM
      >Maybe I'm missing something here.  Is diSessa saying there is no force between your butt and the chair? 
     > Both are affected by gravity.  Your butt is 'pressing down' on the chair (gravity) and the chair is exerting an equal and opposite        > force on your butt."   Otherwise you would fall to the floor.  

  No, I did not mean that diSessa had directly said this. But his quoted statement, when applied straightforwardly to described situation, may mislead the students into believing this. A far more accurate statement would be: The net zero momentum flow through a closed surface only means the zero NET force on the object enclosed by the surface. But it does not say anything about possible interaction forces between a certain part of this object and the environment. Similarly, the zero momentum flow through an open surface element (e.g., through the seat of the chair on which I am sitting) only means that the action-reaction forces between the objects on the opposite sides of the surface are opposite and equal in magnitude. It does not say anything about the actual magnitude. To find it, in the above example, I need to insert the scale between my butt and the chair. But this takes us back to the "old-fashioned" direct measurement of the force. 

   > Likewise, the chair is affected by gravity but the floor is exerting an equal and opposite force on the chair.  And on and on... the        > floor and support walls, the walls and the foundation... etc. My students always ask where does it end?

It does end with the whole planet Earth taken into the picture.
 Moses Fayngold,, NJIT      
 





On Monday, October 14, 2013 1:15 PM, John Denker <jsd@av8n.com> wrote:
 
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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