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Incidentally, this is not the model I am really working on. This just reduces the issue to the simplest case. The two things I am confused on is how to hook up a solid circle to a spring foundation at the center to leave it free to rotate (is my approach workable?) and how to weld the beam to the solid circle in 2D so it would maintain connection point and connection angle to the boundary of a solid. I am looking for general hints and guidance, not specific model debugging.

I continue to be confused and so I am trying to make even simpler models. Here a solid circle (designated as a rigid domain) is hooked up via its center to a spring foundation (k=1N/m). A 1 N force is then applied in the +X direction to the bondary point at the 6 o'clock position. My expectation is that the circle will rotate by 90 degrees so that the point to which the force is applied moves to the 3 o'clock position. The entire circle will also move by 1 m in the +X direction to reflect the overall force balance. At that point a stationary solution would be reached. This is not what I am seeing. The solution does converge but the numbers and the displacement directions are nonsensical. Once again, any guidance is appreciated.

OK, so I decided it would be best to start by implementing "spring foundation" explicitly because I cannot get any answers about how to hook it up to a rigid body while allowing rotations. The idea would be to implement "body load" which would be proportional to the negative displacement of the center of mass of the circular rigid body. I can find the center of mass by creating the Mass Properties node and then creating the appropriate variable. But I still have a new problem. When I run MBD simulation, where my rigid body is subject to a uniform force in the x direction, I can see from Displacement and Velocity plots that the body does indeed move. But if I run Derived Values->Global Evaluation for mass1.CMx then it says that the denter of mass is the same for all time points. How does one compute the center of mass in a meaningful way? Please help.

So after many days of banging my head against the wall, I solved the first problem. For anyone who tries to do the same thing, the solution is to split the circle into a circle and a concentric annulus with a hinge joint in between. It helps to make cuts in the small circle which start at origin. Comsol does not give us the center of the circle as a "point" but it can be defined by making these cuts. However the cuts need to be symmetric so that the hinge center could be defined relative to the center of mass if you want the center to coincide with the actual geometric center of the circle and annulus. This model checks out. If I apply some force to the lowest point on the outside, then the point rotates by 90 degrees and the spring foundation gets stretched by an appropriate amount. Also damped (or undamped) oscillations can be seen. All looks fine. ~ And now I am onto problem #2. If I take this geometry and attach a beam to it (see the model), then the beam does not rotate with the annulus, despite the beam-solid and solid-beam connections being in place. In fact the model solves fine but if I try to plot the location of the end of the beam as a function of time, I get an error. I know everyone here enjoys watching others spend days and weeks figuring trivial stuff out but maybe this time around someone would deign a hint. Please. On a related note, does anyone have a model which shows a non-trivial usage of "multibody dynamics", "beam", and "solid mechanics" modules together? I looked through examples but they are not informative. Comsol has an example of a table with legs but it is useless: legs are beams but their connections are to a shell and not a solid. More importantly, they are not simulating this table being thrown up and then landing on a flat surface, with stress calculations and kinematic calculations coupled. So any model which shows solid mechanics analysis of a collision with solids and beams in the mix would be very welcome.

More debugging issues. I added a small rigid domain to the end of the beam because it was complaining about the lack of mesh near the end of the beam. So now if I apply an off-center point load to the main shape (on the outer boundary of the annulus) then the annulus rotates and everything behaves fine... except that the new small rigid domain I added does not move. It is as if it is not connected to the main shape by the beam or anyting else. Sure enough, if I apply volume load to the small rigid domain, it moves with constant acceleration in the direction of the force, also as if it were not connected to the main shape. I have put in beam-solid connections on both ends of the beam. What am I doing wrong? Why is Comsol not "seeing" the beam?

Ah, so there may be a versioning issue here. I am using Comsol 5.2a Apparently in 5.3 you can make beam attachments to rigid domains. In my version there is no such option. So what was the workaround before 5.3?