Re: [Phys-l] Errata for FCI?

[Philip Keller <PKeller@holmdelschools.org>]

I don't believe that this item belongs on the FCI.  The buoyancy/air pressure issue is very subtle and requires more time to address properly than I have available in a first year class.  And the item requires meta-thinking: I have to guess what issues the test-writer wants me to consider.  Am I to assume that there is a thin layer of air between the book and table?  What if my book cover is made of smooth floppy rubber and the book was pressed against the table?  Can I ignore that suction-cup effect if no one is lifting the book at the  moment?  Or if there IS an air layer, do I count that there is a tiny difference in the pressure above and below?  

The purpose of a test question is to differentiate between the group that answers correctly and the group that does not.  I don't know the "right" answer to this item, and I don't know what I learn about a student who picks wrong.

Other items on the FCI do in fact tell me something.  For example, when a student says that an object moving upward in free-fall has a force on it that gradually diminishes, I learn that that student has a hazy understanding of the difference between velocity and force.  So that item is useful to me.  This item is not. 

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Chuck Britton
> Sent: Wednesday, November 03, 2010 9:55 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Errata for FCI?
>
> Let me try to collect my thoughts here in public.
> Nomex underwear is ON!
>
> Any floatable object that doesn't float is subject to a net downward
> 'sticky' force that prevents the floating. (This force can be - as we
> have seen - quite complicated - but it exists).
>
> It seems to me that (the same) buoyancy acts on any submerged object.
> I would like to think (but am quite willing to change my thinking)
> that the nature of the buoyant force doesn't change as the density of
> the object varies from less than, to greater than, that of the fluid.
> .
> At 9:34 AM -0400 11/3/10, chuck britton wrote:
> > Thank you for resurrecting this 'sticky' subject.
> > I have yet to see the concise argument against D.
> > If we are going to use the term 'Bouyancy', do we need a definition
> > other than the Archemdian 'weight of displaced fluid'??
> > What might that new, PC (Pedagogically Correct) definition be??
> >
> > .
> > At 9:00 PM +0800 11/3/10, carmelo@pacific.net.sg wrote:
> > > Actually, I am still looking for a more appropriate microscopic
> > > definition on buoyant force. Besides, it seems that many physics
> > > teachers and physics education researchers have misconception on
> > > buoyant force. Perhaps, there should be errata published for numerous
> > > research papers on Force Concept Inventory (FCI)? Please refer to the
> > > question below.
> > >
> > > A book is at rest on a table top. Which of the following force(s)
> > > is(are) acting on the book?
> > > 1. A downward force due to gravity.
> > > 2. The upward force by the table.
> > > 3. A net downward force due to air pressure.
> > > 4. A net upward force due to air pressure.
> > >
> > > A 1 only.
> > > B 1 and 2
> > > C 1, 2, and 3.
> > > D 1, 2, and 4.
> > > E none of these, since the book is at rest there are no forces acting on it.
> > >
> > > In many papers on FCI, the answer is D based on the definition that
> > > buoyant force is the magnitude of the weight of fluid displaced by the
> > > body. Perhaps, analysis should be carried out again on FCI based on
> > > the answer C instead? Some of you may prefer B to be the answer? :-)
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