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I solved this for 3DOF compliance on the red sphere with a chain of prismatic joints and links with <implicit_spring_damper> behavior and <dynamics> elements defining <damping>, <spring_stiffness> and <spring_reference> elements.

I used 10 gram links with reasonable symmetric inertia matrix elements and made the spring-dampers critically damped for the moving mass, and it seems pretty okay.

I had some trouble with the sphere snapping to the global origin, but only when I had the damping set way too low by mistake. Not 100% sure whether or not <implicit_spring_damper> was also helpful, but seems like there's a lot of discussion about having this turned on by default, which makes sense to me to avoid instabilities.

It's a pretty verbose way to define a 3DOF spring-damper but it works okay.

I found this discussion:

https://answers.gazebosim.org/question/24001/is-there-a-floating-joint-in-gazebo/

This suggests

A floating joint in Gazebo (SDF) is just no joint 😉

But I think the ability to define dynamics on various axes means that "no joint" and "a 6DOF joint" are not identically equal. It'd be cool to have some 2DOF and 3DOF translational joints that do this job more tersely.

We've already got revolute2 and ball joints for the rotational equivalent I guess.

A 6DOF bushing type joint on which one can define dynamics would be nice, but I think I know how to do the job I wanted to do now with the tools we have.