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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?



If uniform gravity is the only force acting we can quickly show that K + mgh is a constant (given some reference h=0). I don't really follow how the words "reside" and "extract" apply. K and mgh are simply mathematical byproducts of the integration of Newton's 2nd.

F=ma

In 1-D:

-mg = m dv/dt = m dv/dy dy/dt = m dv/dy v

-mg dy = m v dv = m/2 d(v^2)

hence (after integrating) mgh + K = constant, implying that the mechanical energy constancy is just a restatement of Newton's 2nd for this single force system. There is no need for "residing" or "extracting" - just a constancy of the sum.

Bob at PC



-----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-----
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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