[ODE] newbie question: easiest way to create an 'immobile' pendulum

[vincent ngai]

Hi, what is the easiest way to create an 'immobile' pendulum?
Its not exactly immobile...I want to be able to move the top body by
applying a force, but should a force not be applied to the top body,
the pendulum body swing below it should not be able to move the top.

The way I've thought of is the find the force from the joint acting on
the top body and apply an inverse force thus canceling the movement of
the top body...However
this seems overly complicated for what would be a seemingly trivial task.


Thanks
-- 
Regards,
Vincent