View Full Version : Re: Torque or Moment or Angular force or Turning action...

Paolo De Leva
11-26-2007, 10:50 PM
Dear subscribers,

I received three very interesting messages. Those from Drs. Bill Sellers and
John P. Paul are attached below. Notice that Paul calls "turning action" the
rotational analogue of force. In my opinion the word "action" is appropriate
in this context, because Isaac Newton used it as a synonym of force (see his
third law). The third message was from Donnie Curington, an engineer who
works in USA for a large mechanical engineering (ME) consulting firm. He
confirmed that mechanical engineers in USA use the terms torque and moment
exactly as defined by Paul (see below) to refer to different components of
the "turning action". He also explained that, analogously, the components of
a force are given specific names, such as "axial load" and "shear". This
terminology is also used in structural engineering (SE).

Please consider these equations:

axial load + shear = force (in ME and SE)

normal force + friction = force (in physics)

bending moment + torque = moment (is this used?)

moment + torque = turning action (in ME)

In my opinion, the fourth one is questionable, because the meaning of the
word "moment" should not be restricted to a component of itself (the
"twisting moment" or "torsion moment" or torque). Similarly, it would be
questionable to call "force" the "normal force" or "load" the "axial load".

With my kindest regards,

Paolo de Leva

Department of Human Movement and Sport Sciences,

Istituto Universitario di Scienze Motorie

Rome, Italy

--------------- Original message ---------------
From: Bill Sellers [mailto:wis@mac.com]
Sent: 12 Nov 2007 21.57


Torque is certainly a much more recent term than moment of force. According
to the OED it dates to the late 19th Century with the earliest quote they
give being 1884 and an interesting reference to an 1899 paper suggesting
torque as the unit equivalent to one dyne centimetre in the French system. I
don't know if it ever took off in that context.

Torque is much more widely used in the UK. Torque-wrenches can be bought in
any hardware store and the engine specifications for cars always quote the
torque, whereas I'm not sure that many non-specialists would know what a
moment of force was, and there are certainly no moment-wrenches.

Moment for all its prior art is just too ambiguous a term when used on its
own and moment of force is clearly just too much of a mouthful for everyone
(which is why it always get shortened to moment) so I think you are right in
supposing that it will become the progressively less commonly used term. It
reminds me a bit of electric potential which everyone refers to as
voltage... Language, even scientific language changes, and in this case I
think for the better.

A third word might happen if the right standards body insists. The
geological period the Tertiary was destroyed a few years ago and the new
official terminology is slowly taking over as referees and editors start
enforcing the new words.




Dr. Bill Sellers Email:


Lecturer in Integrative Vertebrate Biology Skype: wisellers

Faculty of Life Sciences Tel. 0161 2751719

The University of Manchester Fax: 0161 2753938

Jackson's Mill, PO Box 88 Mob: 0785 7655786

Sackville Street, Manchester, M60 1QD, UK http://homepage.mac.com/


--------------- Original message ---------------
From: John Paul [mailto:john.paul@strath.ac.uk]
Sent: 15 Nov 2007 22.39


Recognising that the deformation of a prismatic body due to a turning action
in a plane including the longitudinal axis of the body (bending) is not the
same as the deformation due to a turning action in a plane perpendicular to
the axis of that body (twisting) it is logical to designate the two turning
actions differently. Thus Moments produce bending and Torques produce

At least one of the Committees of the International Standards Organisation
has accepted this distinction.

If one uses the one word "Torque" for both situations the clumsy distinction
between Bending Torque and Twisting Torque (or is it a Torsion Torque?) is

I suspect that those using Torque for everything are hard core dynamics
specialists whereas most dynamicists whose speciality is mechanics of
materials find it useful to distinguish Moment from Torque.

With kind regards,

John P. Paul