View Full Version : R: Torque or moment about a joint

Paolo De Leva
11-07-2007, 05:55 AM
Dear subscribers,

for some people, there is a difference between moment of a force and torque.
For instance, Prof. John Paul about 20 years ago wrote a letter to the
editor of the Journal of Biomechanics, maintaining that the word torque
should be used to indicate a particular kind of moment of a force.

For sure, the expressions "moment of a force" and "moment of a couple" (of
forces) are both correct. Moreover, the word "moment", in physics and in
mathematics (statistics), has a precise general meaning, which explains also
the expressions "moment of inertia", or "moment of area", or "second moment
about the mean" (i.e. the variance).

However, there are university physics textbooks in US (for instance the
widely used textbook by Reisnick-Halliday), in which the expressions "moment
of a force" and "moment of a couple" are never ever used. They use the word
"torque" to indicate exactly the same quantity. Please notice that I am not
referring to textbooks of biomechanics or engineering. Reisnick-Halliday's
text is widely used for general physics courses. And we need to refer to
physics in this case because the quantity was defined by Isaac Newton when
biomechanics and engineering did not even exist.

Since many scholars in the field of physics use that convention, we cannot
state that it is wrong, based on a different convention used for instance by
mechanical engineers. You can at most express a preference.

Moreover, those who prefer using the word "torque" with the above mentioned
meaning, typically understand the alternative terminology, and vice versa.

So, although I would love to have a standard terminology in physics, and
although I hold Prof. John Paul in high esteem, I conclude that this is not
a big issue.

By the way, there are people who love using the word "couple" to express the
same concept. Didactically, this terminology is extremely questionable, and
I doubt it can be found in physics textbooks. It is important to stress the
point that a single force can start a rotation. You don't need a couple for
that. Some students doubt it. We should insist that this is possible, rather
than just converting the single off-center force into an equivalent
force+couple system. A mathematically correct, but didactically misleading
trick. We should start teaching the effect of a single off-center force and
then that of a couple of forces, rather than vice-versa. We should try and
explain why a single off-center force produces a rotation if applied on a
free rigid body. It's not very easy, but useful.

With kind regards,

Paolo de Leva

-----Messaggio originale-----
Da: * Biomechanics and Movement Science listserver
[mailto:BIOMCH-L@NIC.SURFNET.NL] Per conto di Dale Cannavan
Inviato: marted́ 6 novembre 2007 18.00
Oggetto: Torque or moment about a joint

Dear Subscribers,

Can anyone explain / define the differences in the meaning of Torque and
Moment. It is often stated that they are synonymous; however, some
suggest they are different.

Thanks for your time,

Dale Cannavan

Ph.D. Candidate

Centre for Sports Medicine and Human Performance

Department of Sports Sciences

Brunel University

West London