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Re: [Phys-l] Errata for FCI?



A non-physicist might ask why the air above the ceiling in the classroom contributes to the atmospheric pressure in the room if "Pressure at depth is due to the weight of a column of fluid above that depth".

A student in a classroom is obviously not holding up the weight of the air directly above the roof of the school - the roof is.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Ken Caviness
Sent: Wednesday, November 03, 2010 10:58 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Errata for FCI?

Ok, how's this for an explanation that a non-physicist might
understand:

Briefly: Pressure at depth is due to the weight of a column of fluid
above that depth. Any submerged object will experience different
forces of the fluid pushing on its upper and lower surfaces, and the
net force of the fluid on the object will be upward and equal (in
magnitude) to the weight of the displaced fluid = difference in the
"two columns".

Filling in one or two gaps:

1. Place a flat plate of negligible weight and area A horizontally at
some depth d in a fluid. The weight of the column of fluid above the
plate is pressing down on the plate from above. If everything is in
equilibrium, the pressure on the top face of the plate = the pressure
on the bottom face and so the force on the top = the force on the
bottom. We conclude that the pressure varies with depth, but the
pressure force is "inward perpendicular" -- directed straight at the
surface affected -- not downward only.

2. Now place a box with top and bottom areas A in the fluid. The
horizontal forces due to fluid pressure cancel out in a pairwise
fashion. But the downward pressure force on the top face and the
upward pressure force on the bottom face do not cancel out, being due
to weights of different amounts of fluid: the column of fluid above
the top face and the column of fluid that would have been above a flat
plate of area A at the position of the bottom face if the object had
not been there. The difference between these is the net force of the
fluid pushing on the object. It is clearly directed upward and is
equal in magnitude to the weight of the fluid which would have been
between the top and bottom faces of the object, had the object not been
there: the weight of the displaced fluid.

Ken Caviness
Physics
Southern Adventist University

-----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 10:23 AM
To: Forum for Physics Educators
Subject: 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?
:-)
>_______________________________________________
>Forum for Physics Educators
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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

_______________________________________________
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