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

[Scott Goelzer <s.goelzer@comcast.net>]

On Jul 19, 2007, at 8:54 PM, John Denker wrote:

> On 07/18/2007 09:50 PM, Dan Crowe wrote in part:
> > >  an active force is a force exerted by an animate organism
>
> On 07/19/2007 04:29 PM, Scott Goelzer wrote in part:
>
> > Ideally, all students would come to realize that there are no passive
> > forces.
>
> OK, now I'm even more confused than when we started.

Sorry,

> Am I to conclude that all forces are exerted by an animate organism?

Of course not. I am discussing what student believe, not what is  
reasonable. Getting students to realize that inanimate objects that  
do not appear to move,bend, or flex can exert forces is stunningly  
difficult. Getting them to realize that the forces that inanimate  
objects exert are not somehow different is sometimes impossible.

> I'm not trying to be difficult;  I just can't see any other way to
> interpret what I'm being told.  Is there a typo somewhere?
>
> The more I learn about passive forces and active forces, the more I
> want to stay away from them.

Right, if I could banish student preconceptions by simply stating  
they are wrong I would. I do not use active or passive terminology in  
class. I never ask my students to categorize forces as such. But when  
I listen to how they explain and understand forces, passive and  
active are what come out of the discussion.  They may not call them  
by those names, but those names are a good model of how students view  
their universe. If I don't understand how they think, I cannot work  
on the misconception.

> > The concept is not DOA
> > since it exists for students.
>
> Why should we let the inmates run the asylum?

Running the asylum by stating that the problem does not exist doesn't  
work either.

> I'm not shy about
> confronting misconceptions when necessary ... but I see no reason
> to think this active/passive business is necessary.

It is necessary since it exists in the minds of students.

> Discussing a
> previously inchoate misconception will often consolidate it rather
> than dispelling it.  Not to mention the time spent discussing it.

Then we are back to students who memorize and regurgitate what they  
are told to pass exams and believe what they always have believed.  
They form no _functional_ mental model. Each part of physics becomes  
more incomprehensible than the last. The conception of active or  
passive forces is not inchoate in the mind of a student; it is  
entirely real and well entrenched. This is never more clear than when  
a student can recreate a FBD for a demonstrated situation, but then  
reverts to what they believe when given a different but parallel  
situation. So far, the only luck I have had is to create enough  
cognitive dissonance so that a bright student can really start to  
question how they look at the world. Demos and socratic dialog are my  
best methods.

Scott




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