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Paolo De Leva - Sport Biomechanics, Rome,italy
09-14-1995, 03:30 AM
Dear subscribers,

I am forwarding a message by David Lemmon, with the subject "My
insight into the work/energy discussion". It was sent directly to my address
immediately after it had been suggested by Hinrichs to summarize this
particular discussion, and before it was eventually decided to keep it
public.

As well as the interesting contents of David's message, you may enjoy
his successfull effort of diplomacy. These two things together make this
message a good contribution. I hope it will help to reach the final
agreement that I have been seeking (not as patiently as David,
unfortunately). Of course, that does not mean I fully agree on everything
David wrote.

Maybe there's someone else who wants to post her/his contribution. If
you are one of them, please allow me to insist suggesting you to make always
clear "by what" and "on what", whenever they use the word "work". I feel
this is crucial. We have been discussing several types of work (internal,
external, PARTIAL, TOTAL, on the water, on the weight, by the swimmer, by
the water, etc.), and it is very easy to get mixed up.

I would like to apologize again for my too passionate replies.
Biomechanics has dozens of branches. There's one thing that makes us a
single comunity: our sound common backround in physics (the "mechanics"
component of BIO-mechanics). My intention was only to promote the
developement of this common background, and underline the absolute need of
its soundness. Although I did that too aggressively, and apparently in some
cases I was not very successfull, I still hope that the overall effect was
positive.

I still have a high esteem of many of you, but now I have a feeling
that several people just needed to participate, directly or indirectly, to
this public discussion about simple basic physics, though I might be wrong.
Let me know if you agree. Personally, I would have preferred not to
participate in this discussion. I just felt forced to do so, and made it
somewhat unwillingly, for the reasons I tried to explain above.

Paolo de Leva (DELEVA@Risccics.ing.uniroma1.it)
Sport Biomechanics
Istituto Superiore di Educazione Fisica
Rome, Italy

P.S. I am sorry for the delay. I was too busy last week.


Here's David's contribution:
----------------------------

************************************************** **************************
Date: Thu, 07 Sep 1995 15:27:39 -0400
From: drl10@psu.edu (David Lemmon)
Subject: My insight into the work/energy discussion
************************************************** **************************

Dear Dr. Paolo de Leva:

I would like to send this message to the original poster of this topic, but
by now I have lost the address of that person, so I am sending it to you.
If you would like to post this response to BIOMCH-L, please feel free.

Like many others, I have been giving the matter considerable thought. I
believe the reason there are such heated emotions on both sides is that both
sides are correct, in a sense. Let me explain.

One side seems to be saying that as the swimmer swings his (or her) arms
around and kicks his feet, he is performing positive work on the water in
which he is immersed. The other side claims that the whole process is
cyclical and thus, if there is no net motion of the swimmer (or motion of an
attached, pulleyed weight), there is no net work being done. I can see
where each is coming from.

Consider a cyclical biomechanical problem in which a person is pressing
against a spring and then releasing back to the unloaded condition, say,
with simple harmonic motion. During compression, the person's hand is doing
positive work on the spring. However, during release, the spring is doing
positive work on the person's hand [NOTE by P. de Leva: although this is
true, I believe that here it would be better to highlight that during
release the work done BY the hand on the spring is negative; that's why at
the end total work on the spring is zero, if you neglect the trifling
effects of friction and spring inertia].
We all agree that work is force times displacement. If we
consider both the force and displacement to be positive on the "in" stroke,
the force on the return stroke is still positive, while the displacement is
negative. Thus, there is negative work being done by the hand on the return
stroke. The net work performed by the hand in this cyclical problem is
zero. Some call these forces "conservative", referring to the principle of
conservation of energy.

Now, let's replace that spring with a damping device such as an automobile
shock absorber, with the same simple harmonic motion. The person's hand
performs positive work in the "in" stroke. However, on the return stroke,
the damping device is not performing work on the hand. Because the signs of
the force and the displacement become negative, the work is still positive.
Thus the net work is positive. So these forces are called "non-
conservative" forces.

In viewing these two scenarios, I believe that the tethered swimming problem
is related to the latter, rather than the former. The forces applied by the
swimmer are non-conservative, meaning, if the swimmer relaxes, the water
will never push the hand back to where it was before. Thus, I would say
that, even if the swimmer is held stationary by a tether and performs
cyclical motion, he (or she) is still performing net positive work.

I hope this is helpful information. I believe in the existence of truth, and
hope that through this discussion, we may somehow come a little closer, at
least in the realm of biomechanics.

Regards,
=====================================
David R. Lemmon, Ph.D.
Center for Locomotion Studies (CELOS)
10 IM Building
The Pennsylvania State University
University Park, PA, 16802 USA
Phone: 814-865-1972
FAX: 814-863-4755
=====================================