View Full Version : Re: Torque or moment ? about a joint

11-08-2007, 01:11 AM
Dear all,

I find a subtle difference between torque and momentum.

From my remembering of physics books, momentum (with respect to a
point or to an axis) is proportional (through the inertia) to the
rotational acceleration. That is it induces a change in rotation of
the body. In this respect we can also represent the effect of the
momentum as a torque which is a vector oriented with the rotation
axis and has magnitude equal to that of the momentum.

From my point of view torque can be considered an abstract
equivalent representation of the momentum, that is the same torque
can be generated by a shorter arm and a larger force or viceversa.

From the biomechanics point of view, when analyzing a body joint
from a global point of view we may talk as (net) torque on the
articulartion, but if we want to talk at a more detailed level I
would prefer to read about the momentum of the different muscles
(which is implictly due to stretching force plus momentym generated
by the offset between insertion point and point of istantaneous rotation).

Just the same as the difference between 'jam' and 'marmelade'


Alberto Borghese

At 15.18 08/11/2007, Adrian Smith wrote:
>The Oxford English Dictionary has TORQUE (noun):
>"The twisting or rotary force in a piece of mechanism (as a measurable
>"the moment of a system of forces producing rotation."
>Adrian Smith
>Leeds University Library
>+44 (0)113 3435531
>-----Original Message-----
>From: * Biomechanics and Movement Science listserver
>[mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Altinus L Hof
>Sent: 08 November 2007 09:41
>Subject: Re: [BIOMCH-L] Torque or moment about a joint
>Dear All,
>That is an interesting discussion, indeed, about possible differences
>between moment and torque.
>To a great extent this is a 'jam or marmelade' discussion. It just
>happens that English has two words for (about?) the same thing.
>I have the privilege to teach biomechanics in the Dutch language, and
>Dutch has the word and concept 'moment' but no equivalent of 'torque'.
>For all I know it is similar in German (and other languages?). The
>problem is then, when students are to write something on mechanics in
>English, they ask which of the two terms to use. My answer is: the
>meaning of both words is the same, but you better use 'moment'.
>In English or US textbooks as a rule one of either term is used, and it
>is stated that both terms are equivalent. A quick scan of my bookshelf
>showed that 'torque' is used by
>- Hamill & Knutzen
>- Kreighbaum & Barthels
>- Mc Ginnis
>And 'moment' by
>- Winter
>- Tozeren
>- Robertson ('Introduction..')
>- Robertson et al. ('Research methods..')mostly
>- Chaffin & Andersson
>- Hibbeler (engineering text)
>- Prentis (,,)
>- Zatsiorsky (Kinetics of human motion)
>The latter reference, for me in difficult cases the ultimate authority,
>suggests to reserve the term 'torque' for the moment of a couple ( p.
>The special thing of such a moment is, that it is a free moment and does
>not depend on the location of the axis with respect to which it is
>calculated, different from the moment of a single force.
>In the example of Young-Ho Kwon: with a screwdriver you apply a torque,
>with a spanner you apply a moment to the screw or bolt. In biomechanics
>therefore, in my opinion 'moment' is in 99% of the cases the correct
>term, and no major misunderstanding will result if it is used in the
>remaining 1%.
>At Hof
>Center for Human Movement Sciences
>University of Groningen
>PO Box 196
>9700 AD Groningen
>The Netherlands
>tel. (31) 50 363 2645
>fax (31) 50 363 3150
>e-mail a.l.hof@med.umcg.nl
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Prof. Ing. Alberto Borghese
Department of Computer Science - University of Milano
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Email: borghese@dsi.unimi.it Skype: borgheseA

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