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Re: + work in cyclic motion - D. Lemmon's contribution

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  • Re: + work in cyclic motion - D. Lemmon's contribution

    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

    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

    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 (
    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: (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.

    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