Re: [Phys-l] ? passive force of constraint

[Jack Uretsky <jlu@hep.anl.gov>]

	It seems to me that this kind of distinction is incompatible with 
the spirit of free body diagrams, which are a basic imnplementation of 
Newton's 3d law.  In the dialog labs I referred to earlier, we have 
students draw diagrams of moving, or accelerating bodies using colored 
pencils.  There is a color for force, a color for acceleration, a color 
for velocity, a color for displacement.  But all forces are the same 
color, to emphasize the point that they can be (vectorially) added to each 
other when acting at the same point.  The attempt to identify forces 
according to their agency seems to me to be a throwback to Aristotelian
thinking and should be eschewed from discussions of dynamics along with
earth, air, fire, and water from discussions of the basic composition
of matter.
					Regards,
						Jack


On Sun, 22 Jul 2007, carmelo@pacific.net.sg wrote:

> Quoting John Denker <jsd@av8n.com>:
>
> > Let me explain why I needed clarification as to where list members
> > were coming from.  Possibly my first exposure to the active/passive
> > business was in Arons, _Teaching Introductory Physics_.  I quote
> > from page 76:
> >     "... it turns out to be helpful for students to distinguish
> >     between two classes of forces, designated as 'active' and 'passive,'
> >     respectively."
>
>
> In a sense, there is still no "intelligent design" on terminology, but
> active force could have "evolved" from impressed force, living force
> or innate force etc. Active force could refer to the "impressed force"
> which is an *action* exerted upon a body, in order to change its
> state, either of rest, or of uniform motion in a straight line. (We
> can distinguish action and reaction.)
>
> On the other hand, passive force could refer to resistive force, drag
> force, or
> constraint force etc. It should be a good practice to distinguish the
> force to be either active force or passive force, driving force or
> resistive force etc. Just like we have active device/element and
> passive device/element in electric circuits etc... or even active
> gravitational mass and passive gravitational mass...
>
> Alphonsus
>
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