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-----Original Message-----_______________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Wednesday, December 15, 2010 1:35 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?
Ok, maybe prove was not a good word. Demonstrate or show might be better.
But in a sense it is a proof by saying if gasoline contains energy you
should be able to extract it without adding anything else. But that is not
possible, so the idea that it contains energy is disproven. But of course
like all proofs it requires assumptions at the beginning.
The idea that energy is in something has already been well established by
the time students take a physics course. They are told gas, food... all
contain energy. The caloric content of food is already on the packages.
But this idea needs to be broken, so the students can understand it is the
food+Oxygen that contains the energy, and not the food alone. They also
have been told in various textbooks that the chemical bonds contain energy,
Yeeeuch. The same text will say in one section that the bonds contain
energy and you have to "break" them to get the energy. This is completely
false. But in other sections it will say that you get energy when bonds are
formed. Doesn't anyone look to see if the text is consistent. OK, there is
the example of nuclear energy where you have to get out of a local potential
energy minimum to get to the unbonded lower minimum. But the idea that
breaking bonds releases energy is not true in chemistry. And of course
bonds do not "break" they merely stretch and weaken. The word break
conjures up rubber bands breaking which is the wrong analogy.
As to the word non-conservative, perhaps the word dissipative might be
better as long as it is qualified to indicate the mechanical energy went
somewhere else. When you dissipate fumes, you merely spread them out or
exhaust them to somewhere else. At the intro. course level perhaps heating
and non-heating would be better. So friction and air resistance are heating
forces while gravitational, elastic... are non-heating. That would help
make the connection in student minds as to where the energy goes.
The published energy ILD does show how mechanical energy is conserved, or
not conserved, but it does not use the word non-conservative. But you have
to always ask the students where the energy went, and to make a bar chart
showing where it went.
Perhaps students should be asked to come up with a word to describe this.
They might come up with better ones than we can dream up.
John M. Clement
Houston, TX
On Dec 15, 2010, at 11:22 AM, John Clement wrote:
... As to conservative and
non-conservative forces, that is a fairly destructive
nomenclature. ...
Yes indeed. The term "potential force" would probably be better than
"conservative force." But the word "potential" has another meaning, as
in potential customer, etc. As someone stated earlier, we are
prisoners of language. I think this came up was causality was
discussed. What does the verb "to prove," (in John C.'s first
sentence) means in physics. Yes, I know what it means in mathematics.
Ludwik
http://csam.montclair.edu/~kowalski/life/intro.html
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