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

------------------------------------------------

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

------------------------------------------------

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

------------------------------------------------

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

------------------------------------------------