[ODE] Torquing the joint

[jnilson_99@yahoo.com]

> When I apply the torque it makes the body behave
> like a propeller, turning
> through the center of mass - but instead imagine
> that you have a body
> attached to a string and you spin the body in a far
> arc. The center of
> rotation is not at my hand that holds the string.
> Clearly these aren't the
> same.


I know exactly what you're talking about. The classic
example of what you want is best exemplified by the
difference between a propellar and a pendulum.

A propeller spins about it's center by applying ONLY a
torque.

Whereas a pendulum is just like a propellar except it
has an additional linear acceleration applied to it at
the center of mass, gravity in the case of a pendulum.
The torque is applied by the joint holding the
pendulum to a fixed object.


To get the motion you want you need to apply both
torque and linear acceleration. I think angular joint
motors do this implicitely for you.


john

--- Henri Hakl <henri@cs.sun.ac.za> wrote:
> When I apply the torque it makes the body behave
> like a propeller, turning
> through the center of mass - but instead imagine
> that you have a body
> attached to a string and you spin the body in a far
> arc. The center of
> rotation is not at my hand that holds the string.
> Clearly these aren't the
> same.
> 
> Another example: apply torque to your lower arm by
> twisting it manually in
> the middle with your other hand, and due to the
> nature of your body this
> torque is propagated to your upper arm. This is fun,
> for sure, but what I'm
> looking for is the "torque" that needs to be applied
> to just let the arm
> pivot at the elbow (imagine you're lifting dumbells)
> - and the upper arm
> stays motionless.
> 
> I think it is true that the overall angular momentum
> on the body(part) may
> be the same, but here I'm looking for a more complex
> interplay of forces.
> Any ideas?
> 
> 
> ----- Original Message -----
> From: "Nate W" <coding@natew.com>
> To: <ode@q12.org>
> Sent: Wednesday, December 04, 2002 11:16 PM
> Subject: Re: [ODE] Torquing the joint
> 
> 
> > On Wed, 4 Dec 2002, Henri Hakl wrote:
> >
> > > I've put together a little articulated system
> and now wish to apply
> > > torque forces to various parts of it. But I'd
> like the torque forces
> > > to act at the joints/pivots, rather then through
> the center of mass of
> > > each object. Any suggestions on how this can be
> accomplished?
> >
> > It's my understanding that the effect is the same
> either way.  Don't
> > worry about it. :-)
> >
> > --
> >
> > Nate Waddoups
> > Redmond WA USA
> > http://www.natew.com
> >
> >
> > _______________________________________________
> > ODE mailing list
> > ODE@q12.org
> > http://q12.org/mailman/listinfo/ode
> 
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