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Re: [Phys-l] F causing a or Delta-V causing I



This reminds me of the answer to an oral question at UCB related by my Sr. E&M instructor. The woman disst. student, whose name I've forgotten, was asked to describe how to "obtain" a potential of one MV. [I think the examiners were interested in the description of a machine, e.g. a Van de Graaff.] She answered push one amp. thru a one meg resistor.

bc

p.s. below is what I was going to post, but thought better. I pray my reconsideration is not a mistake.

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

I don't have much trouble w/ equality. I think I understand it's character, i.e. is R, S, and T. What I have trouble w/ is that "F=MA" accurately describes the Physics, i.e. I think it's nothing more than a means of calculating a result. I'm stuck at level 3 (Tim's), as I don't think "=" is appropriate for more than calculation.

Since I've grown beyond ABBA, I though I'd appeal to authority. Lotsa results to <"force causes acceleration"> from academic sites, also stuck at 3, but this one is very interesting:

http://singapore.cs.ucla.edu/LECTURE/lecture_sec1b.htm

Perhaps when I've had time to digest it, I'll progress.

bc, preparing for a Paleo. expedition this week end.




John Denker wrote:

Folkerts, Timothy J wrote:


IMHO, "F causes ma" is a very helpful rung on a climb onward toward
understanding physics.


Can you explain why it is helpful?

The idea that equality is reflexive, symmetric, and transitive "should" be
introduced in 7th grade (possibly sooner), and "should" be pretty well
consolidated before students reach HS physics ... let alone college physics.

So please explain why introducing a new (and incorrect) asymmetric notion
"F causes ma" has even a chance of being easier than using the existing
symmetric notion "F equals ma".

Equality is a much simpler notion than causation. Why use something complicated
as a rung on the climb to something simpler? That violates the most basic
pedagogical principles.

Also: the idea that equality is reflexive, symmetric, and transitive is so
fundamental and so important that reinforcing it in physics class is valuable
... and conversely, undercutting it with a needlessly asymmetric re-intepretation
of what equations mean is destructive.

I understand that students throw around words like "cause" and "effect", but
do they really have the slightest inkling of the technical meaning of the
words? They also arrive with nontechnical notions of "heat" and "force"
and who-knows-what-all-else, but we do not normally accommodate those notions;
we do not let the inmates run the asylum.

The choices are
a) Accommodate and reinforce wrong ideas about cause and effect,
b) teach the correct meaning of cause and effect, or
c) avoid those terms and stick with established ideas of equality.

I judge (c) to be easiest and most correct; (a) to be almost as easy
but destructive, and (b) to be difficult distraction, beyond the scope
of the typical course.



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

Robert Cohen wrote:

I had written:

Thus, it is clear that current through the element causes a voltage across it (Delta V=IR) and not the other way around. :)


to which John Denker responded:

Sorry, that's not clear at all.


And I'm sorry I wasn't more clear. I wasn't serious when I said "not
the other way around".

<snip>

Therefore, if you believe in proof-by-example, it must be "clear" that the equation V = I R should be interpreted as a statement that R causes V.


I suppose I should've used that language instead of the ambiguous smiley
face. So, for completeness, this is what I should have written:

Therefore, if you believe in proof-by-example, it must be "clear" that
the equation V = I R should be interpreted as a statement that I causes
V. (can I still add a smiley face?)

To get back to the original discussion, it is important to recognize (as
Jack Uretsky pointed out a while ago) the difference between writing an
equation that reflects the dependent/independent variables and writing
an equation that represents "cause/effect". My point is: just because
you can "control" R does not mean that R "causes" V. ____________________________________________________
Robert Cohen, Chair, Department of Physics
East Stroudsburg University; E. Stroudsburg, PA 18301
570-422-3428; www.esu.edu/~bbq _______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
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