Re: Torque

[Leigh Palmer <palmer@SFU.CA>]

> The terms torque and moment are often interchangeably used. Is there a
> distinction between them? I have always seen text often referring to torque
> of a couple, and moment of a force (less often torque of a force). Can
> someone pls comment.

It is acceptable to refer simply to the torque associated with a couple.
The torque, a vector quantity, is the proper measure of the strength of
a couple. It is not acceptable to refer simply to the torque associated
with a force. Such torques must be referred to a point in space. Perhaps
it is easier to remember that if one uses instead the phrase "moment of
force" to emphasize that distiction.

Whenever I use a symbol for a torque (or angular momentum, or moment of
inertia) I *always* affix a subscript to the symbol as a reminder that
the reference point should be specified. A couple is, indeed, a special
case, but I affix a subscript anyway, and then demonstrate the
independence of reference for that case. By the way, a couple is not the
only sort of force combination that has this property. Three forces
which form a closed triangle will generate a reference independent
torque perpendicular to their plane, and four or more forces which have
no resultant (and hence might form a non-planar polygon) also generate a
reference independent torque.

Leigh