View Full Version : Re: Positive Work in Cyclic Motion

Leonard G. Caillouet
09-05-1995, 02:54 PM
Peter Davidson made two points that were close to being correct but are
in fact examples of his own mistaken description of the problem of
describing work. He states that "work is a form of energy..." In fact
work is the transfer of energy. When describing the work done by an
object, or the work done on an object one must define the question of
interest carefully. As Peter stated, the system must be defined. His
mistake can be illustrated most simply by the example of the object
thrown straight up which falls to rest in exactly the same location. The
problem is that the object doing the work we are interested in measuring
was not defined carefully. There was indeed no net work done on the
object thrown BY ITS ENVIRONMENT, but there certainly was work done by
the thrower on the object. The thrower displaced the object against the
force due to gravity and changed its mechanical potential by the distance
above its resting position it was lifted. The fact that gravity caused
the object to be displaced in the other direction does not make the work
done by the thrower zero.
Similarly, the fact that the kinetic energy transfered to the
particles of water is ultimately dissipated to the environment does not
make the work done on the water BY THE SWIMMER zero. This work would
seem to be the transfer of energy that is of interest in this
experimental condition.
To quickly summarize, the discussion of work must include the
definition of the bodies of interest because work is not energy but the
transfer of energy. If the bodies of interest are not described in the
statement of the problem we can expand the system infinitely to the level
of the universe and the problem becomes one of "understanding entropy."
Careful use of terms such as work and energy is essential.
Considering them to be equal would be like using heat and thermal energy
to mean the same thing. A final note to Peter on this point: You
suggested that there may be confusion on the list concerning "effort",
"work", and "energy". I suspect that few of us are confusing effort and
energy, but the latter terms may be used carelessly. The more important
lesson is to carefully define the objects of study between which energy
is being transfered. It is one of the first lessons in mechanics that
Newton would have taught, I'm sure. It's certainly one that most of us
who studied physics or mechanics had to learn.


Leonard G. Caillouet
Louisiana State University
MS Candidate, Dept. of Kinesiology