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Re: [Phys-l] Errata for FCI?

On 11/3/2010 8:00 AM, carmelo@pacific.net.sg wrote:
M. Horton wrote:
I agree that the original question may not have been asking what it
intended> to ask. So, I'll ask what I think the original
questioner was intending to> ask because I've long had the same
question. (Wow,>that was confusing.)> Where does the buoyant
force come from?
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? :-)


Best regards,
Alphonsus
I see angels dancing....
It would be easier to agree that there is compression on the book due to the ambient air pressure.
The materials of a book are usually permeable, so its gross volume is not the contributor to buoyancy that one might suppose, and in a similar way, the under surface of the book is not air tight.
There is a down force on the table under the book which is not quite so large as it would be if the book and table were in an evacuated container. This is usually ascribed to the buoyancy due to the air excluded by the stuff of the book.

Brian W