[ODE] Adding a torque about a hinge

[Gary R. Van Sickle]

> I had a look at dJointAddHingeTorque and found that it simply 
> adds a torque in the direction of the hinge axis to one body 
> and the reverse to the other body of the hinge.
> 
> Could anyone please help me understand how this works? When 
> the torque is applied to each body, is it applied about their 
> center of gravity? If so, how does this translate to a torque 
> about the hinge?
> 
> Thanks,
> Lukasz.

This tripped me up as well when I first started out with all this rigid body
stuff.  While a force applied to a point breaks down into a force component
plus a torque component applied at the center of gravity, this does *not*
happen with torques.  When you apply a torque to a body, there is no "at a
point", it is just around an axis, and the "translation" from that axis to
the parallel axis running through the CG is 1:1.

-- 
Gary R. Van Sickle