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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 Philip Keller
Sent: Wednesday, December 15, 2010 1:39 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?
If I lift a block from the floor and place it on the desk, does it have
potential energy? I sure teach that it does. But I can't extract the
potential energy without pushing the block off the desk. So does that
mean the energy "resides" partially in me?
-----Original Message-----better.
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
But in a sense it is a proof by saying if gasoline contains energyyou
should be able to extract it without adding anything else. But thatis not
possible, so the idea that it contains energy is disproven. But ofcourse
like all proofs it requires assumptions at the beginning.established by
The idea that energy is in something has already been well
the time students take a physics course. They are told gas, food...all
contain energy. The caloric content of food is already on thepackages.
But this idea needs to be broken, so the students can understand itis the
food+Oxygen that contains the energy, and not the food alone. Theyalso
have been told in various textbooks that the chemical bonds containenergy,
Yeeeuch. The same text will say in one section that the bondscontain
energy and you have to "break" them to get the energy. This iscompletely
false. But in other sections it will say that you get energy whenbonds 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 localpotential
energy minimum to get to the unbonded lower minimum. But the ideathat
breaking bonds releases energy is not true in chemistry. And ofcourse
bonds do not "break" they merely stretch and weaken. The word breakbe
conjures up rubber bands breaking which is the wrong analogy.
As to the word non-conservative, perhaps the word dissipative might
better as long as it is qualified to indicate the mechanical energywent
somewhere else. When you dissipate fumes, you merely spread them outor
exhaust them to somewhere else. At the intro. course level perhapsheating
and non-heating would be better. So friction and air resistance areheating
forces while gravitational, elastic... are non-heating. That wouldhelp
make the connection in student minds as to where the energy goes.conserved, or
The published energy ILD does show how mechanical energy is
not conserved, but it does not use the word non-conservative. Butyou have
to always ask the students where the energy went, and to make a barchart
showing where it went.this.
Perhaps students should be asked to come up with a word to describe
They might come up with better ones than we can dream up.than
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
as"conservative force." But the word "potential" has another meaning,
mathematics.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
_______________________________________________
Ludwik
http://csam.montclair.edu/~kowalski/life/intro.html
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Forum for Physics Educators
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