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I have noticed that if I link and item with a physics model to an item with no physics model, the default behavior seems to be to wipe all physics models from the linkset permanently.

For example, I linked a piece of mesh ground (full physics) to a patch of grass (no physics) and now I walk straight through the ground. I have tried with multiple objects with the same result. It does not seem to matter which prim is the root prim is and unlinking does not bring the physics back, nor does copying the affected piece. I have also tried letting the sim reboot to no effect.

I can understand if physical objects cannot be linked to no-physical objects, but objects should retain their physics models if you unlink them.

Are you referring to non-Phantom or to non-Physical objects?

As a general rule, do not make objects Physical if it can at all be avoided - the existing ODE physics engine works but such objects can create a lot of load on the sim and that will degrade your world's performance.

Oz, my experiences are following:

- at every OpenSim grid mesh objects are able have got only that physics, which is chosen when mesh object is uploaded. Linking can "broke" the physics to something, I do not know what precisely, looks for me like "lowest" or random physics for object. But linking cannot change the physics, not for "other" physics, from other object, neither prims nor other meshes.

- if your physics is "broken", you can fix it and return to uploaded physics:
-- in Edit - Features, Edit linked parts, choose root prim of mesh, switch Physics Shape Type to None, switch again to Prim (which is default, but it is needed to be switched, don't ask my why:)
-- choose every other "prims" in mesh, do the same, but with None - default is None, switch it to Convex Hull, switch back to None.

I do not know why, but this procedure is sometimes needed for meshes after upload or unlinking. Definitelly needed after unlinking mesh, even if you unlink and link the same parts of mesh again. It is like meshes work for me... It is very different from SL behavior.
The process is described for Singularity, but I guess other Viewers have got same options.

Sorry it took me a while to reply, I have been trying to pin down the issue and its a tricky one to describe

So it seems that if you link an item with physics to an item that is either phantom, set to "none" under the physics shape tab or a model that never had a physics model uploaded to is (i.e - at point of upload) the physics shape is lost. I believe this is the default behavior in SL too. You have to have one physical linkset and one non-physical.

The weirdness begins when you unlink the items. Both stay "phantom", even after changing the physics shape type in the build menu multiple times.

To clarify Ilan I am referring to the physics model given to items at upload so that you can walk and bump into an item realistically

oh, and sorry Zuza, thank you for the tips! I will have a play around and see if I can get any positive results.

Oz, not sure, if I can explain it precisely:)
When you unlink items, which was uploaded with some physics, one of them probably has (/is able to have:) the physics, which was uploaded. Seems logic it would be root prim, but it is not irreversible rule in OS:) For me, when I unlink an object, one of prims has physics, others not. Maybe it is more complicated, when you got plenty of "prims" in the linkset.
Physics are kind of "linked" to root, when uploaded (shrinked/expanded to root prim), so this works unexpectedly, for me and maybe for you too:)
I am usually even able to unlink mesh, change root prim, link it back, and some magic happens - physics is still OK, after that "absurd" switching of Shape Type:)

The big difference between OS and SL is when you upload other mesh object in SL, with completely different physics, and it is linked as root, physics work as uploaded with root object, independently on physics of child prims in linkset. So you can use several different physics for one mesh object, just by changing (invisible) root prim linked to mesh. You can even use the usual, rezed prim as physics.
At least it is how it works for me:) And it is still quite unexpected sometimes, hard to say if it is a feature or a bug. I have never found such detailed description for mesh physics in OpenSim.