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Re: The Rise and Fall of Simple Machines.



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Dear Teacher:

For the second time you are asking me to deal with something
else that I wanted. My original question was "where am I
wrong?" in making a generalization from what we did in the
lab and what I read about Joule's work. It was my own
exercise in critical thinking and you say absolutely nothing
about it (after the implied rejection). Why is it so?

.. energy (mechanical or thermal) can be said to be
"work done on a system" or "heat supplied to it".

You also totally ignored my comment on "accepting things on
faith". It was your idea, I responded and you totally ignored
the comment. It is very important to me that subsequent
messages address the issues raised in previous messages, and
that we keep the discussion on the level of what I am supposed
to know so far. We did not learn anything about physics of
engines yet and I have no idea what is Y in your unfamiliar
equations (see below).

The remark about engines was not part of the dialog as you will
see if you scroll backwards. I'm just trying to elicit your
reason for believing that compressing a spring is anything like
storing latent heat, presumably also by doing work on something
(e.g. rubbing an ice cube).

"The usual suspects" you should look up, already!

My observations were made on the basis of simple machines
and calorimetry. Why are you focusing on heated springs?

I did the calculation because I couldn't believe the number
you came up with. Where did you get it?

Why did you mention the spring in the first place? I know
nothing about heated springs and I am sorry I started to make
bad guesses. The only connection between heat and energy I
know about is that 4.18 J of work raises the temperature by
nearly the same amount as 1.00 calorie of heat. "Work is not
lost, it exists in the form of heat". That is just another way of
saying what was quoted above.

We'll work on that problem some more after we clear up your
initial false premise.

Sorry for bringing the initial sentence back again; I really
want to know what is wrong with it it. Can it be explained
in my language or should I just accept your criticism and
wait till I know more physics? Perhaps you are right,
students should not try to act as scientists; critical thinking
at my level should be strongly discouraged. Learn what
others found first and try to be creative later.

Respectfully, H.S.S

I include your original question (which you have apparently
forgotten) below. You draw a parallelism between elastic
potential energy stored in a spring and latent heat:

A simple machine used to compress a spring corresponds to
a calorimetric setup in which "latent heat" is accumulated,
perhaps in a solar house for subsequent release. Thus energy
(mechanical or thermal) can be said to be "work done on a
system" or "heat supplied to it".

Such generalization may surface naturally in a Socratic
postlab dialog. Should we immediately say NO, NO, NO?
How should the dialog continue to correct the misconception?
I will be happy to play the role of a student, if somebody
takes on the role of a teacher.

I take the first statement (that elastic potential energy is
somehow similar to latent heat) to be the misconception to be
relieved in this exercise. After we take care of that we can
go on to correct other difficulties, but for now, let's stick
to the question at hand.

Did your teacher neglect to explain to you that the elastic
property of springs reflects the elasticity of the material
of which springs are made? If so, you have been short changed.
The topics should be coupled closely (Hecht does that right).
Go forth and learn the intensive version of Hooke's law:

F dL
--- = Y ----
A L

Leigh