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



On 07/18/2007 09:50 PM, Dan Crowe wrote:

Many students in introductory physics courses believe that only active
forces exist. The following is a typical statement: "The table doesn't
exert a force on the falling ball: It simply gets in the way." Physics
education research has shown that addressing this preconception helps
many of these students to better understand normal force, tension and
other passive forces.

We agree that this misconception exists. We agree that addressing it is
helpful.

Tangential remark: I see this misconception as one element of a large
bundle of misconceptions. Other elements include the pre-Galilean
notion that objects at rest remain at rest, and objects in motion tend
to come to rest.

It is also connected to innumerable problems with the terminology.

When it comes to the table "getting in the way", there are at least
two ways of explaining the situation to students:

a) Explain it in terms of a force of constraint. Explain that
in terms of Hooke's law, using a large spring constant k and
a small deflection x. This is easier to visualize if you replace
the tabletop with a not-very-taut rubbery drumhead, as discussed
at http://www.av8n.com/physics/causation.htm

b) Explain it in terms of passive versus active. For reasons
discussed below, this is IMHO in no ways better and in many
ways worse than option (a).

.... an active force is a force exerted by an animate organism and a
passive force is a force exerted by an inanimate object. Not all
passive forces are forces of constraint. Gravitational and macroscopic
electrostatic forces are passive forces.

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.