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



Hi all-
I think that some exercises with free-body diagrams can help students rid themselve of the spurious distinction between active and passive forces. In Scott's example, one can start with pictures of a weight being held up by a weightlifter, and a seight resting on a flat surface. Now, in each case, have the student draw a free-body diagram of the weight. If the diagrams are different, some careful leading questions can be used to lead the student to making the diagrams identical.
Moral: as already pointed out, a force is a force is a force.
Regards,
Jack


On Thu, 19 Jul 2007, Scott Goelzer wrote:

Your "clip on a cat" is a perfect example of why students mentally
divide forces into two unnecessary categories; a student can
mentally take the place of the cat and empathize with the feeling of
supporting the clip. They cannot be the table or an inanimate object -
no matter how many theater classes they have taken.

A student will correctly identify that a weight lifter is pushing up
on the bar, that's an active force.
The bar resting on the floor is passive and whether the student
identifies an upwards force from the floor is going to be very
personal and inconsistent.

A student sitting passively in an accelerating car identifies a
backward force as being responsible for their motion (too many assign
this as the force of inertia - another type of passive force). The
same student holding a rope being pulled in a rolling chair gets the
FBD correct - they were active in creating the force.

Active and passive forces are useful because they provide a basis for
discussing with students what they are experiencing and believing -
hopefully leading to 'force unification'. The concept is not DOA
since it exists for students. Telling students only one kind of force
exists is similar to attempting a religious conversion by just arguing.

I spend much time in class on converting students from viewing a
normal force such the floor pushing them upward from passive to
active. They see the deflection of the floor using an optical lever
and connect their motion in the room to the movement of the laser. Or
when the optical lever is connected to the wall they indirectly see
the wall bend and push back. For many, the floor remains immovable
and passive no matter what demos and discussions are presented.
Ideally, all students would come to realize that there are no passive
forces.

The physics teacher explaining forces is not much different than the
missionary telling the polytheist natives that they are wrong.

Scott




*******************************************
Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
sgoelzer@coebrownacademy.com
*******************************************





On Jul 19, 2007, at 3:34 PM, John Denker wrote:



Animate organism? Really?

It looks like things are worse than I thought. I suspected there
was something problematic about the definition of passive force,
but I never dreamed it revolved around distinguishing animate
forces from inanimate forces.

We agreed there was a problem, but IMHO this cure is worse than
the disease. Do we really want to teach kids that the physics of
animate organisms is different from the physics of inanimate objects?
AFAICT that just substitutes new misconceptions for old. And the
new misconceptions are worse than the old.

1) By way of illustration, just for starters: I have here a thin
flat plastic clip that came off a bread wrapper. According to
the definition given above, setting it on top of my desk gives
rise to a passive force ... whereas setting it on top of my cat
gives rise to an active force.

Do you really want to hang your hat on the animate/inanimate peg?
That idea seems DoA chez moi.

2a) Turning from the definition to the examples, we see tension
listed among the passive forces. Suppose I tie a string to a
block, and pull on the other end of the string to make the block
slide across the floor. According to the definition, this seems
like an active force, because of the central role played by the
animate organism (me). Also, why should we care whether it is
active or passive? The force is a force. It does what it does,
whether you call it active or passive or neither.

2b) Rather than dragging the block, I just hold my end of the
string stationary, to provide a force of constraint as the block
undergoes more-or-less circular motion. Is this active or passive?
Why should we care whether it is active or passive?

2c) Suppose I move my hand a little bit, so that the block undergoes
more-or-less cycloidal motion. Is this active or passive? Why
should we care whether it is active or passive?

3) Similar scenarios can be used to make a mockery of the idea
that normal forces are passive.

===========================================

Not meaning to put words in anyone else's mouth, let me hypothesize
that the mention of animate (as opposed to inanimate) might have
been an attempt to express the idea of /intention, volition/ or
/causation/. This would allow us to make sense of the cat scenario,
since the cat was not supplying much intention with respect to
supporting the weight of the bread-clip.

Before we go too far down that road, let me emphasize even though
such a restatement would make the active/passive distinction more
logical, it would also make it highly unscientific.

As previously discussed (05/05/2006 10:56 AM), many thoughtful
folks consider the year 1638 to be the epoch, i.e. the dawn of
science as we know it. As Galileo pointed out, physics must
say _what happens_. Physics need not (and usually does not)
say _why_ something happens. You are free to wonder _why_ if
you want, but usually that falls under the heading of metaphysics
or philosophy ... not physics.
http://www.av8n.com/physics/causation.htm

Newton went to school on Galileo (literally and figuratively).
He expressed this important idea in the famous words "hypotheses
non fingo".

As always, IMHO the primary, fundamental, and overarching purpose
of the course is to teach the kids some high-level thinking skills
... to teach them to think logically and scientifically. That
is far more important than any domain-specific knowledge about
this-or-that force law. From this point of view, the idea of
classifying forces as animate versus inanimate strikes me as a
reeeally bad idea. It would set science back 400 years.

======================

To return to where we started, it seems to me that the table
that "gets in the way" can be satisfactorily described as a
force of constraint.
http://www.av8n.com/physics/causation.htm#sec-kx
Therefore the table is not a persuasive motivation to introduce
"passive" versus "active" forces.

This leaves us with the question: Is there any good reason to
introduce passive versus active forces? I haven't seen one yet.

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley