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

11-07-2007, 06:18 AM
Dear Hamid and all,

It won't make any difference. That + ranch you use to loosen or fasten a
bolt has two forces acting on the arms. Each one contributes to the system
both linearly and angularly.

dp/dt = Sum (Fi) - force
dH/dt = Sum (ri x Fi) - moment or torque

where p is the linear momentum. In the case of a couple, the net force
(dp/dt) becomes 0. dH/dt is of course of sum of the torques (moments)
produced by the forces. Instead of using both arms of the + ranch, some
people hold the long axis (axis of rotation) of the ranch with one (left)
hand and push one arm with the other (right) hand. In this case the net
force still becomes zero while only the force from the right hand produces a
torque because the left hand applies a force through the axis of rotation:

dp/dt = Sum (Fi) = 0
dH/dt = Sum (ri x Fi) = r(R) x F(R) + r(L) x F(L) = r(R) x F(R)

My background is astronomy (physics) so I don't know how torque and moment
are differentiated from one another in Engineering but I've never seen any
kind of distinction between torque and moment. I looked at Wikipedia before
I wrote this posting and they were defined exactly in the same way, too.
Anyway, to me torque is simply the angular equivalent of force as the
equations above show.

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

> -----Original Message-----
> From: * Biomechanics and Movement Science listserver
> [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Dr. Hamid Rassoulian
> Sent: Wednesday, November 07, 2007 12:54 PM
> Subject: Re: [BIOMCH-L] Torque or moment about a joint
> Hi again,
> Since I am tempted to make a more daring contribution would
> you consider having acquired a puncture whilst driving and
> you want to change the wheel.
> Also consider that you have at your disposal one of those
> general purpose spanners that look like a + with a socket
> at each end. So to take off the bolts of the wheel you would
> apply the spanner to the bolt and with both hands (one on
> either side, one pushing down and one pushing up) you would
> apply a "twisting" force or torque to the bolt. However, if
> you were to use one hand only you would be applying a turning
> moment to the bolt.
> thanks
> Hamid