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Re: [Phys-l] motion problem

They still sell these submarines for a few dollars. You fill the inside
with baking soda, and watch the sub rise and fall in water. Demo can be
messy and a little unpredictable at times.

Anyone ever do the raisin demo? Put a carbonated beverage (preferably
clear) in a beaker. Predict what will happen if a few raisins are added to
the liquid. Do it, observe, and explain the results.

(Most of) The raisins will rise and fall as the carbonation bubbles attach
to the raisins, increasing their buoyancy (decreasing their density).
Bubbles pop when they reach the surface, and the raisins sink. Process
repeats for a while. Can be done with different sodas at different
temperatures, etc.

It's pretty neat to watch, and this activity can be done/analyzed in
science classes at any level. Physics is PHUN!


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
I am currently 65 years old and hearken back to a parochial elementary
education where science simply wasn't taught. I learned about buoyancy
from an old toy I sent away for that was a submarine operated by a tablet
of Alka-Seltzer. A compartment in the hull was initially flooded and the
submarine sank. As the Alka-Seltzer tablet dissolved the gasses released
displaced the water and the submarine surfaced. I read everything I could
get my hands on to find out just what made it work. I basically
understood buoyancy before my first high school class in general science.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony
Lapinski
Sent: Monday, November 02, 2009 7:24 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] motion problem

I was never taught buoyancy in any of my science courses, from elementary
through graduate school. I teach it in my honors physics class. Great
topic with neat demos!

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
People!

I find it curious that buoyancy is so ignored. I read in elementary
school about the Greek scientist who, on using it to solve the
content problem of the ruler's bauble, ran naked. Now evidently it's
even ignored by the authors of an applicable use in the current issue
of the "The Physics Teacher".

bc not only a pedant, but a critic, and he wonders if he's turning
into a Hake

p.s. When did you-all learn about buoyancy? One form of
intelligence is the ability to use information from different fields
or topics to solve problems, no? Should "we" be "teaching"
intelligence or just rote learning?

p.p.s. I'm reading Banesh Hoffmann "The Tyranny of Testing". Banesh
shows quite effectively the failure of multiple-choice, sentence
completion, best paragraph selection, etc. tests. The majority of
his examples are from the ETS.

From the cover blurbs (Dover ed. 2003 with an additional Jacques
Barzun forward -- original 1962):

"Hoffmann's complete and well-documented account of the failings and
dangers ... illustrates the inherent flaws in ... achievement tests.
It demonstrates the inadequacies of multiple-choice testing, in which
candidates simply choose answers and need not justify their replies,
revealing the tests' inclination to reward superficiality rather than
subtlety and creativity." [bc's emphasis]

---------------------------------------------------------

This question seems fine to me, with A being the correct answer. It
seems
this question would be posed during a unit on dynamics, with things like
buoyancy coming later in the semester. To me, choosing E seems
pedantic for
our purposes here, namely, teaching basic dynamics to high school
students.

John Reed
Saint Mary's College H.S.
Berkeley, CA

----------------------------

I think it's a great question, because it affords a "teachable
opportunity". If I were Jason, I would first explain it's not a
"plain old" multiple choice question, but an opportunity for the
class to think carefully about the question. They must supply a
diagram and discuss it. If they successfully do (not including
buoyancy, but including the absence of drag), I'd give nearly full
credit. Those who include buoyancy (E), etc. get a bonne note! [sp.
corrected] Doing so also affords a discussion of horological
pendula. The better ones, not isolated in a vacuum chambre, include
an aneroid bellows compensator. So technology meets physics.
[better: physics informs technology]



bc pedant.

The original post:

On Oct 28, 2009, at 1:24 PM, Jason Westfall wrote:

Hi All,

I had this as a question on an exam, and had a hard time
convincing the
kiddos of the correct answer. Could someone help me to explain it
in a
different way?
Thanks.
J. Westfall, Sonora High

A rock is thrown up from the earth's surface. Which one of the
following
statements concerning the net force acting on the rock at the the
top of
it's path is true?

A. The net force is equal to the weight of the rock
B. The net force is instantaneously equal to zero newtons
C. The net force's direction changes from up to down
D. The net force is greater than the weight of the rock
E. The net force is less than the weight of the rock, but greater
than zero
newtons.


-------------------

BTW, if E is a distractor, tho I think you-all agree it's "more
correct", please explain it's distraction.

_______________________________________________
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