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Re: Torque or Moment or Angular force...

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  • Re: Torque or Moment or Angular force...

    Dear subscribers,

    Ton van den Bogert suggested to close this discussion "unless someone has
    .. actual new material". I agree with him. I submitted this contribution
    while his message was still being processed (8 November 2007 22.04), and he
    posted it as an attachment to another message with a modified subject header
    ("one more...") and format. I have cleaned and completed the text and
    resubmit it now with the correct subject heading and format. My contribution
    meets Ton's requirement, because it describes an alternative point of view
    (see subject header). I suggest that those who want to comment on this do
    that by sending a message to me. After two weeks from now, I will post on
    BIOMCH-L a compilation. This way, we will not bother too much those who are
    not interested in this topic.

    Paolo de Leva.

    -----Original Message-----

    Dear subscribers,

    Ton van den Bogert is right: "it would be great if we could agree on this"
    (i.e. on using "moment" rather than "torque", as suggested by John Paul).
    However, in this case we have two separate and widely accepted standards.
    Standards, not opinions. Here is an alternative point of view, explained in
    ten points:

    1) There has been a widely known precedent: SI (Systeme International
    d'Unites) versus the British system. The SI was born in the 18th century;
    its mother was the French revolution, it became adult in the late 20th

    2) Today it is not yet completely accepted in US, but it will. May be in 10,
    50, 100 or more years, but it will. Indeed, nobody can honestly deny that
    the SI is the best system, not even those who keep using a different system.
    They do so just because they are used to. The change is too expensive for
    them: a desired good that they cannot afford to buy.

    3) I am not going to compare BIOMCH-L with the France of the 18th century.
    This is not the point.

    4) The point is that "torque" versus "momentum" is a war that nobody can
    win. Both standards have strong weaknesses! (see for instance the
    etymological notes in my previous messages; in addition, consider that
    "torque" comes from the Latin "torquere" = "to twist").

    5) To win a war, a third, less questionable name is probably needed. For

    "angular force" or "rotational force", as opposed to "(linear)
    force" (notice that in the late 18th century Joseph Louis
    Lagrange introduced the "generalized force", 6 numbers
    including force and moment of force), or
    "rotor" (if John Paul says that a torque produces only torsion,
    and a motor produces motion, please indulge me and let me say
    that a rotor can produce a rotation, even though this word is
    already used to indicate the rotating part of an engine), or
    "gyrator" (etymology similar to "rotor"; already used in
    electronics to indicate an impedance inverter)

    5b) However, the war will never be won by "nihilating the enemy". No new
    terminology (not even the word "torque") will ever completely substitute the
    expression "moment of a force", because "(nth) moment of a quantity
    about..." is used in physics to indicate the multiplication of a quantity by
    a distance or power of a distance, as in:

    (First) moment of mass, used to compute CM position.
    (Second) moment of mass = moment of inertia.
    (First) moment of the quantity of motion = angular momentum.
    (Second) moment of area, used by structural engineers to
    estimate an object's resistance to bending.

    5c) The expression "(nth) moment of ... about" is also used in mathematics
    to indicate a similar concept [see "
    (mathematics)"]. For instance:

    "Second moment about the mean" = variance.

    6) Do we know how many scientists (not only biomechanists) are used to use
    or chose to use the word torque as a synonym of moment in their studies
    (including physicists)? Every year, thousands of (native and foreign)
    students learn physics in USA, or studying textbooks written by US
    physicists. The influence of USA universities on the scientific community is

    7a) I suspect that "moment" was the word initially used by everybody. My
    guess is that "torque" was later adopted by some influential physicists
    because they did not like the aspecificity and ambiguity of the word
    "moment" (it comes from the Latin "movimentum" = "movement", it also means
    "instant"; what has that got to do with rotation?). This change was
    intentional, not casual!

    7b) There was also another important change, which is probably related:
    Newton called "quantity of motion" the quantity that we now call (in
    English) "linear momentum". And "moment of the quantity of motion" the
    quantity that we now call "angular momentum". Not difficult to imagine that
    this change was welcome by everybody! Again, "quantity of motion" is too
    generic. As you see, the rationale is similar.

    7c) The conflict between two different usages of the word "torque" is not
    enough to discourage physicist or engineers. It exists also for the
    expression "moment of inertia"; many structural engineers keep referring to
    the "second moment of area" as the "moment of inertia" (although relating
    the word inertia to an area is definitely questionable)! For solving this
    conflict, rather than just cancelling the improper usage, they invented this
    horrible specification: "area moment of inertia" versus "mass moment of

    8) Notice that the SI prescribes the name of the unit ("Newton metre"), but
    as far as I know it does not give a preference about the name of the
    quantity ("moment of force" or "torque").

    9) A representative international committee (ISO) is the only organization
    that can solve this problem. And most likely, the committee will be mainly
    composed of physicists.

    10) An optimistic observation: at least this is not a very expensive change.
    A new non-questionable name for this quantity may be accepted faster than
    the whole SI. There is no need of a revolution.

    10b) Pessimistic: possibly, other terminological "fixes" are needed in other
    chapters of physics. A change in a single word will not be accepted, if
    other questionable words or expressions (such as "area moment of inertia")


    It does not matter if we agree on using the word "moment". We are not going
    to convince the other half of the word. Terminology evolves, but changes are
    expensive and nobody will ever accept questionable changes; etymologically,
    both "moment" and "torque" are questionable words.

    Moreover, I suppose that the river of history is flowing in the opposite
    direction! We are just advocating a return to the past. We are not starting
    a revolution, but resisting against a revolution started a long ago.

    With my kindest regards,

    Paolo de Leva
    Department of Human Movement and Sport Sciences,
    Istituto Universitario di Scienze Motorie
    Rome, Italy