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Re: [Phys-l] Errata for FCI?



I disagree (but am waiting to be convinced otherwise).
We, as Professional Physicists, can make ANYthing complicated.
As 'teachers' we want to make things easily understood.

What is wrong with teaching that a buoyant force exists that is equal to the weight of the displaced fluid? (Try to keep your answer simple - for MY sake.)
The nature of contact forces needn't complicate the picture (IMHO).

The contact force question is very similar (if not identical) to a student's very thoughtful questioning of how the floor 'manages' to exert 'just the right amount' of Normal Force in every situation.
It's not simple - but seeing the floor as stiff trampoline seemed to help.
Moving the wall with a laser optical lever helps too.
We are comfortable with using the 'Normal Force' concept without worrying over the detailed mechanics of stiffness and displacement of the floor.

I'll ask again - what's WRONG with teaching the 'weight of displaced fluid' concept.
(and try to keep it simple - for us simple minded folk)
.
At 2:06 PM +0000 11/3/10, Philip Keller wrote:
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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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l