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



kwon3d
11-07-2007, 04:09 AM
Dear Dale and all,

Contrary to what some subscribers posted earlier, I don't think 'torque' and
'moment of force (moment)' are any different. I personally have used these
terms interchangeably. I looked at my mechanics books (Analytical Mechanics
by Fowles and Classical Dynamics by Marion and Thornton to name a few) to
make sure but most mechanics books treat moment and torque synonymous.

Defining moment as force couple is definitely incorrect. Moment is the
rotary effect a force causes about the center of rotation in a single force,
a couple, or a group of forces. A couple is just two forces acting in the
opposite direction so that the net force becomes zero with a non-zero net
moment.

Moment of force or torque produced by a force about the axis of rotation can
be defined as r x F where r is the relative position of the force
application point to the axis of rotation and F is the force vector. 'x'
denotes the cross product. No difference here.

A subtle difference that I can think of is in regards to the the first time
derivative of angular momentum:

dH/dt = sum(ri x Fi)

where H is angular momentum.

I beileve we call dH/dt (rate of change in angular momentum) torque, but not
moment of force. It seems 'moment' more refers to what a force causes on the
object of interest while torque can also be used for the effect the object
experiences.

My two cents!

Young-Hoo
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Young-Hoo Kwon, Ph.D.
Director, Biomechanics Laboratory
Texas Woman's University
+1-940-898-2598 (Office & Fax)
ykwon@mail.twu.edu / kwon3d@kwon3d.com
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