Re: transfer of momentum

[Bob Sciamanda <trebor@VELOCITY.NET>]

I realize that the concept of momentum flow is put to good calculational use
by mechanical engineers in static stress situations.  But regarding John's
description (below) I might remark that N2 says that it is the NET force on
an object which is equal to the time rate of change of the object's
momentum.  To associate a momentum rate of change (or flow) with each of
several forces acting on an object is a bit of a conceptual stretch,
although of no calculational harm.

Perhaps one might distinguish, and associate each individual force with a
"virtual" momentum change (flow) and associate only the NET force with the
actual momentum flow.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.velocity.net/~trebor
trebor@velocity.net
----- Original Message -----
From: "John S. Denker" <jsd@AV8N.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, November 18, 2003 6:53 PM
Subject: Re: transfer of momentum


> On 11/18/2003 09:09 PM, Ludwik Kowalski wrote:
> > . . .
>  > c) Why should a static force experienced by a
>  > person on a floor (it has the same magnitude
>  > as m*g) be replaced by the "flow of momentum."
>  > Is this concept less mysterious that the concept of
>  > force? In which direction is the momentum flowing?
>
> Contact contributes a flow of upward momentum across
> the floor/person boundary into the person.  That is,
> we have +Z flow of +Z momentum.  This is of course
> trivially equal to -Z flow of -Z momentum.
>
> Meanwhile gravitation contributes a +Z flow of -Z
> momentum.  I don't associate this with the
> floor/person boundary;  rather it flows into each
> molecule of the person independently.