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Re: Why work before energy in texts



Bob Sciamanda wrote:

When you correctly use a work-energy theorem (validly derived from
newton's laws) to calculate a numerical quantity you are on the
solid ground of a mathematical model - a testable numerical equality.

When you interpret these numerical results in terms of "particular energy
losses, gains, transfers, transformations, etc among particular objects",
you are in the completely different realm of a conceptual model - a shaky
ground of subjective taste based on metaphorical associations with everyday
real and imagined human experience.

You can do your students no greater service than to emphasize this
distinction when you explain "How you know".


Yes, We should be careful how we say things. The over-
idealized world of only two forces (weight and elasticity)
is a model we use to introduce energy. Problems like "find
the speed of a skier sliding on ice ..." are first attacked
with this model. The established vocabulary is then used
in more realistic models. I used to have a misconception that
"work is a form of energy". It will be easier to describe
this misconception in the thread "ENERGY WITH Q" (a more
realistic model), perhaps during the next weekend.

In general work done against a conservative force
BECOMES POTENTIAL ENERGY ASSOCIATED WITH THAT FORCE
(energy=ability to do work).

If energy is compared to money then work is a monetary
transaction. Work is what a force DOES when it ACTS on
a particle. dW==F*dx is zero when F=0. It is also zero
when dx=0, no matter how large the force. Energy (of a
particular form) "does exist" before and after work is
done. What does it mean "it exists"? It means that all
physical quantities appearing in the expression for
this form of energy are measurable, at least in principle.
But F, appearing in the formula for dW can be measured
only when the force is acting. To eliminate the concept
of work, and to replace it with the concept of "change
of energy" is not acceptable to me anymore. Suppose
such approach is possible. Why is it better?
Ludwik Kowalski