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



I would say that Arons is not always on the mark, and here is probably a
case in point. He wrote a number of very perceptive things, but many
comments were made before there was more evidence from research.

The term passive force is very misleading and is not always accurate. The
normal force is often thought of as passive because it usually does not
contribute to changes in energy, but when looking at the problem of a marble
rolling down and back up a hill, the horizontal component of the normal
force determines how quickly it gets to the final destination. Forces that
are often thought of as being resistive such as air resistance, and friction
can still accelerate objects in situations such as sail boats and people
stepping onto people movers, or even just running.

So when a distinction between passive and active forces is made, students
tend to think of passive forces as being incapable of adding energy to an
object. So friction is always thought of as always opposing motion, which
is clearly not true. Students do not realize that friction with an already
moving or accelerating object can increase the motion of another object.
This distinction reinforces the tendency of students to look at just one
factor, rather than more than one in determining what the force does.
Students need to understand that a force is a force is a force and that any
forces can speed, slow down, or change the direction of an object. The
common notation in traditional books which use different letters such as N
or T for forces reinforces the strong misconception that each force is a
separate type of thing. There is now a move to use F with a subscript for
all forces, and a corresponding move to use E for all energy, also with a
subscript. While notation does not clear up misconceptions, it can be
helpful by sending the right signal.

I would say the terminology passive and active should only be used in
refutational text where you are exposing a student misconception.
Refutational text has been shown to have a small effect in helping students
overcome misconception, but not a large effect.

While circuits can clearly have passive and active elements, this
distinction really does not carry over to forces. Any such usage with
respect to forces will lead to more misconceptions rather than an integrated
concept of force.

John M. Clement
Huouston, TX


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...