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unknown user
11-04-1998, 08:36 PM
Date: Tue, 3 Nov 1998 13:40:39 +0100
From: Josep Solaz Sanahuja
Organization: Universidad Polit˘cnica de Valencia
Subject: Center of percussion
To: BIOMCH-L@NIC.SURFNET.NL

In "Sport Engineering" Vol.1, Number1, September 1998, there is a couple
or articles ("Hurling equipment" G.J. Fahey et al. and "The center of
percussion of tennis racket" H. Hatze) in which appears the 'center of
percussion' concept.
The definition given by Hatze is quite unprecise from a physic point of
view. "It designates that point on the string membrane which, if
impacted by the ball, produces a racket motion which is such that the
reaction impulse at the so-called locus of restraint at the racket
handle vanishes, provided certain rather stringent (and unrealistic)
conditions are satisfied" (sic)
I'd like to know which is the physic definition of center of percusion,
or where can I find it. (I think that it is not a vibration node).
Thanks.
J. Solaz
Sport Biomechanics
Instituto de Biomecżnica de Valencia

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Dr. J. Solaz from Sport Biomechanics of the Instituto de
Biomechanica de Valencia refers to my definition of the center of
percussion of tennis rackets as being quite imprecise from a physics
point of view (see above), thereby suggesting that he is an expert in
the field and therefore competent to provide professional judgement.

Although I am capable of enduring his harsh criticism without getting
too depressed, I must admit being mystified by the fact
that someone critizes a certain definition as being "quite imprecise
from a physics point of view", and, immediately afterwards, asks the
public to provide him with that very definition he is, judging
from his pretentious statements, supposed to know in its precise
formulation already.

Apart from the fact that my descriptive definition of the center of
percussion of tennis rackets is by no means physically imprecise (the
center of percussion and the locus of restraint are, of course,
conjugate points), a substantial part of my paper deals with the
general physical properties of the center of percussion and the locus
of restraint (stationarity requirements, etc.), and the conditions
under which these concepts are valid, including the detailed
mathematico-physical treatment for three-dimensional impacts
(Equations (1) to (6)). Indeed, equation (9) together with the
definition of the symbols and the verbal explanations quite clearly
provides the exact formal physical and mathematical definition of the
center of percussion in relation to its conjugate point, the locus of
restraint.

The only explanation I have for the fact that Dr. Solaz did not
recognize this exact definition is that he must be in possession of
an incomplete journal copy in which exactly the pages 19 - 21,
containing the above mentioned equations and definitions, are
missing. Otherwise, his criticism would not be understandable.

H. Hatze, Ph.D.
Professor of Biomechanics
University of Vienna

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