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Re: [Phys-L] The Make-Believe World of Real-World Physics

Thanks for your thoughtful, kind response. I joined this list to get help
with physics. I don't consider myself an "expert.," even after 24 years of
teaching. I teach in high school, so I want my students to understand the
basics about how the world works. If they wish to explore ideas at a
deeper level, they can always take the AP class or other courses in
college.

In addition, one doesn't need to be an "expert" to be able to teach
physics. Just because one has a Ph.D. does not mean they are a good
teacher or a good communicator. If a student asks a question that I don't
know the answer to, I usually say I'll do some research and get back to
them. I sometimes ask the people on this list.

I wonder -- how does one become a good physics teacher? Do students
evaluate teachers? How do teachers improve? I certainly have come a long
way since my first year in the classroom.

I have students with a wide range of abilities in my class. I treat
everyone the same, whether they are my weakest or my brightest. I never
use condescending/denigrating language. If I did, I'd probably get calls
from parents and lose my job. I want students to learn some new ideas
about the world, and hopefully enjoy the process. I want them to be
comfortable in my class. Physics should be the highlight of a student's
day because the subject is awesome. I do my best to make this happen, and
I'm always learning new things.


Phys-L@Phys-L.org writes:
Hi Anthony,

Your appreciative response to JD's follow up has increased my respect for
you immensely. I agree with you that thought-provoking,
response-inducing,
open-ended questions are extremely valuable, ill-posed or not. In fact,
the incomplete question often induces questions about how to make the
original question more defined.

As a side comment, and I don't mean it to be a blanket indictment because
the asset of a dedicated expert in so many field of physics is invaluable
to those of us needing guidance, I think JD's comments are frequently
condescending and sometimes even denigrating. Most of us are trying to
teach students who will take one physics class their entire life;
therefore, what is most important for them to retain is not the esoteric,
most complete explanation, but rather the comprehensive completeness, the
idea of important relationships, the conservation laws, and the usefulness
of physics.

Keep asking away.


On Sat, Jul 27, 2013 at 7:53 AM, Anthony Lapinski
<Anthony_Lapinski@pds.org>wrote:

Thanks for your detailed analysis. Much appreciated. I've seen many
similar questions in textbooks, test bank books, peer instruction books,
etc. From what you are saying, it appears than many questions like these
are (unintentionally) "ill-posed." And my guess is that many teachers
ask
these types of questions in class (or on tests).

I have not heard from others on this list about their opinions regarding
these kinds of questions, if they are asked, if students find them
challenging, best teaching strategies, etc.


Phys-L@Phys-L.org writes:
On 07/26/2013 07:31 PM, Anthony Lapinski wrote:

I can comment on most of these questions. Some might seem
underspecified,
but I listed them in general terms without all the specifics. Didn't
mean
to confuse anyone. I wonder what questions teachers on this list ask
their
students.

#1: Might have been better to ask what is the acceleration at any
time

True for one definition of acceleration ... but not the other!

(10 > m/s2 downward as gravity is constant).

The ball decelerates on the way up, and accelerates on the
way down. Maybe this isn't the answer you had in mind, but
how am I supposed to read your mind????

I find arbitrary definitions to be very counterintuitive.

#2: Acceleration on incline depends on angle, so concave hill wins
since
ball has highest acceleration at start.

Wrong physics. Initial acceleration does not fully determine the
outcome. A long initial vertical drop gives maximal acceleration,
but is not the minimum-time strategy.

It's a famous problem. It is a calculus of variations problem.
http://en.wikipedia.org/wiki/Brachistochrone_curve
Just saying "highest acceleration at start" is not a valid analysis.
The correct analysis is more complicated than that. Roughly speaking,
there has to be a tradeoff between highest average speed and shortest
path. If you emphasize the initial speed only, you are no more correct
than the student who emphasizes path-length only, and chooses the
straight (i.e. shortest) path.

Just because you know the desired answer and can throw together
some words that make that answer plausible does *not* mean that
your reasoning is correct ... or that students should be able to
follow your so-called reasoning.

Just because your plausible mnemonic verbiage is simple does not
mean that the correct analysis is equally simple.

I find it entirely likely that your students are not as smart as
Jean Bernoulli. However, that does not mean that they are ignorant
of basic physics principles. Stumping them with this problem does
not prove they lack reasonable reasoning skills.

I am quite aware that when the path-curvature is *near* zero, it
is possible to do a qualitative analysis that is less involved
than the full calculus-of-variations formalism ... but it is still
quite a bit trickier than merely saying "highest acceleration at
start".

#3: Car acceleration is constant. Balloon moves forward as it has
less
inertia (like bubble in level with cart/pulley). These are similar.

The original problem statement forgot to specify that the
acceleration was constant. In the form presented to this
list, which is all I have to go on, the original problem
statement was ill-posed.

#5: Both teams pull with same force (related to #4 -- like a
two-person
game). Winner determined by friction on ground. Can show this with
two-person game with different shoes, or one person on skateboard,
etc.

You've obviously never been in a real tug-of-war.

In reality, the winner /might/ be determined by ground
friction ... or it might not. Consider the two-person
game with their feet braced against immovable logs.
Victory might go to the person with more initial arm
strength, or to the person with the greatest finger
strength, or to the person with the most endurance.

No wonder students find physics counterintuitive. I find
wrong answers to be very counterintuitive.

#13 "Three bulbs in series" is not vague, even without a diagram. Not
sure
what the confusion is here. Kids just think electrons leave the
negative
battery terminal, so the bulb closest to this should receive current
first. They forget what is happening at the other terminal and where
the
electrons are in the circuit.

Wrong physics. Electrons have got essentially nothing to
do with which bulb lights receives current first. Any of
the three bulbs could be the first to receive the signal,
depending on the /geometry/ of the circuit, which is quite
impossible to determine from the original word-picture
alone, without a diagram or a verrrrry much more detailed
word-picture.

If it's not obvious what I'm talking about, re-implement
the circuit using coaxial cables instead of bare wires.
The speed of propagation in the coax is well characterized.
The bare wires are not much different in principle, just
less-well characterized. There are logarithmic factors
that change the details but not the basic principles.
The sign of the charge carriers does not enter into the
propagation equations, not at any frequency relevant to
the introductory course.

Whichever bulb is closest to the switch will receive the
signal first. Determining which bulb this is, based on
the original statement, is purely an ESP exercise.

I find wrong answers to be very counterintuitive.

#14: This is asked after I teach about reflection, refraction,
diffraction, and interference.

OK, I'll bite. Is reflection the right answer to the
original question? Is refraction the right answer?
Is diffraction the right answer? Is interference the
right answer? I hope none of the above.

#15: Mirror is fixed (can't be wiggled).

The original problem statement didn't specify that, and
didn't specify a whole lot of other crucial details.
The original problem was very ill-posed.

#16: Kids don't draw ray diagram.

Well, you should teach them to draw the diagram, before
asking them questions about what happens.

Ditto for circuits. Teach them to draw the diagram.

To say the same thing the other way: I see no point
in throwing non-swimmers into the deep end of the
pool. I suggest the the goal is to make the right
answer become intuitive. Asking questions when they
don't have the tools to find the answer is not the
right way to go about it. You can quote the PER
literature about peer instruction; I'll quote some
other literature about the blind leading the blind.

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




--
Kirk Bailey
Never use a big word if a diminutive synonym is as efficacious.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l