Re: [Phys-l] Errata for FCI?

[John Denker <jsd@av8n.com>]

On 11/03/2010 06:00 AM, carmelo@pacific.net.sg wrote:

> 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? :-)

I gotta go with Rick Tarara's approach: just eliminate the question.

We could hold a contest to see who could find the largest number 
of reasons for objecting to this question, but the reasons already
mentioned are more than enough to support totally rejecting the
question.   No matter what you are trying to do, this question is
not the right way to do it.

In particular, if we hypothesize that the intent was to ask about
buoyancy, this is not the right way to do it.  The question embodies
wrong physics in the sense that the buoyancy is independent of
pressure /per se/, as you can see from the following:  Make a plot 
using density and pressure as the axes.  (The contours of constant 
temperature are hyperbolas on this plot.)  On this plot, contours 
of constant buoyancy are parallel to the contours of constant density 
and perpendicular to the contours of constant pressure!

If we hypothesize that the intent was to check whether the students
can visualize the pressure as a scalar field and the pressure gradient
as a vector field ... well, let's just say that the level of the 
question is inconsistent with the rest of the FCI.

Also:  Ask yourself, how would a student in the introductory class
carry out an experiment to /measure/ the upward and downward forces,
and then subtract them with enough precision to determine the "net"
force due to air pressure?   A clever physics professor might be
able to design a differential experiment to measure these things,
but so what?  That's way beyond the level of the rest of the FCI.

=========

I guess there is a difference between FCI and CSI (i.e. Common 
Sense Inventory).

Common sense includes the concept of immateriality.  In ordinary
situations, the effect of buoyancy on books on tables is immaterial.
You could argue that C or D is "better" than B ... but not better
to any significant degree.  The buoyancy in air is so small as to
be unobservable in ordinary situations, *and* there is no practical
reason to discriminate between the "air pressure" and other pressures
acting on the bottom of the book.  Asking about "air pressure" in
this situation is just foolish.

The fact that this august group has been unable to figure out the
intent of the question is pretty strong evidence that the question
should be discarded.