OK my little animation gurus,  I am making an animation that includes an anchored mine.  The chain links will have a positive density so will try and drag the mine down.   But the mine itself will have a negative density and more than counteract the pull of the chain.   Still with me?  

"Why bother with density" I hear you cry, well I want to add a flow force showing the current and want the chain and mine to act realistically. 

I can make the directional force go up rather than down, indicating the effect of the water, but then the chain wants to go up as well as the mine.    So, my question is:

 

How do I have two directional forces and apply objects to one or the other?

Or

How do I set certain objects to have a negative density ( I can't seem to do it with density)

Or

How should I be doing this?

 

A side question.   Why do my chains always break?!  How do I stop that?  Am I setting the force too strong (default) or the material too dense (metal)?

Attached Link: Animation

Heres a short clip to show both problem.   Chain wanting to float and the chain breaking.

Not that I have an answer but You tube says the video is private.

Would flipping it upside down give the look your looking for? Render as image sequence and then flip the images back, right side up?

Nah, not really since I still want the chain links to be pulled by gravity one way and the mine itself the other.  Flipping it just reverses the problem.

Besides in Carrara I can change the direction of force and avoid having to flip.   But I need two direction of forces on different parts of the model.

Quote - Besides in Carrara I can change the direction of force and avoid having to flip.   But I need two direction of forces on different parts of the model.

It sounds like you're running into a limitation of the physics engine, i.e. you can't setup things up to deal with buoyancy.
If there's no way to specify a medium other than what is effectively air or vacuum, e.g. water as required in this case, then finding a way to apply an appropriate force to just the mine would seem to be the solution.

Can you group forces?

I got some help over on the DAZ forum.  You can set forces to only affect certain objects.  So I have one going up and assigned the mine to it and one going down and assigned everything else to that.   Problem solved.  I'm rendering now and will post in a bit.

Attached Link: http://www.youtube.com/watch?v=4Ls8K6zNLIA

Here's the animation test.  This show the mine wanting to head to the surface, but the chain pulling down.  

 

Can't remember why I wanted to do this now.  I'm sure it was important.

not sure if this is your problem with the chain, but when I had chains break it was because the links was touching before the sim started

 

as for your tower ! did you turn bounce to very low (off even) that solved my fly apart wall

I'll give that a go.  thanks.