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Re: CONSERVATION OF ENERGY




On Wed, 23 Jul 1997, LUDWIK KOWALSKI wrote:

Is it not true that by writing K=R*x (A) is effectively saying that 'the
kinetic energy is converted into work'? But work is not a form of energy.
In the spirit of the first misconception (heat and work are forms of energy)
let me say that 'in this situation first K is converted into work then work
is converted into heat. Thus both work and heat are equal to K. Didn't we
learn that conversions of W to Q can often be 100% efficient?'.

are confirmed. Do not jump, I am not saying this will happen. Just pretend
it happens and tell me if this would be an 'experimental verification' of
the mental picture according to which K-->Work-->Heat. If your answer is

What I really want to know is this. Do we say that the idea (K-->W-->dU)
is nonsensical (a misconception, if you wish) because it does not agree
with other formal definitions, or do we say it is nonsensical because it
is in disagreement with experimental facts?

A hypothesis, K-->W-->Q, is a mental model. It is based on observations
we make with thermometers, speedometers and rulers. Is it verifiable or
not? Is it scientific or not? Is it correct or not? Why yes? Why not?


The fact that the values of K, W, and Q are (nearly) equal does not imply
the inferred relationship. I would say that the model is wrong because it
confuses and obscures what its occurring.

As I see it, there is a net force on the object, no matter its source, and
that force produces a change in K (1/2 m * v^2), not the other way around.
The 2nd Law implies that the change in K equals W (which describes the
action of the force).

The K then shows up as motion and potential energy of the atoms, but I
wouldn't call that energy Q, because Q is used to refer to energy transfers
resulting from temperature differences.

I posted the following some time ago, but it appearently got lost, so I
include it here.

It seems to me that there is a basic confusion in the use of terms in the
original posting that needs to be addressed.

As I understand it, energy is not an agent - it can't do anything. Energy
is, rather, a function of the state of a system that appears to have
great physical significance. If parts of a system interact, energy can be
transfered from one part to another; ie. the interactions result in
changes in the states of the subsystems such that the energy associated
with one decreases while that of another increases.

When the interaction is a force acting on a macroscopic
object, we say that work is done. It is always the force that does the
work; ie. results in the transfer of energy from one subsystem to
another. It is incorrect to say that energy is "the ability to do
work", although I have used that as an initial, working definition in
some non-science courses, before refining the concept. It is true that
an OBJECT that has energy has the ability to do work - ie. to transfor
some of that energy to another object through its interactions.

In classical mechanics, the energy of a (sub)system can be seperated into
the energy of the center of mass and the energy associated with
internal degrees of freedom. For objects like the block and plate that
are being discussed, The kinetic and potential energy are those of the
center(s) of mass, and that of the internal degrees of freedom is
called the internal energy (or thermal energy, etc.) of the objects.

The "force" of friction can be confusing, because it results in an
average force on the CM, which does work (which results in a change in
kinetic energy), but it results from forces between individual atoms that
excite internal degrees of freedom. So, in the case of friction, it is
easy to confuse work and heat.

I hope I haven't rambled too much. I think my main
point is that we need to carefully distinquish between mathematical
quantities that are functions of state, and the agents that produce
changes in those quantities.

One final comment. Although you can include all the energy of a system in
its internal energy, it seems to me that to do so would make the term
superfluous. It is often helpful distinguish between the forms in which
the energy of various parts of system appear.

Al Clark