In almost all 3D physics we treat 3D objects as point masses, and in most cases this is either correct or "close enough". I know of one physical configuration this simplification is totally wrong, but wonder whether others exist (including a specific one I'll mention).
The case I've known about is a hollow sphere (with one or more holes in the surface). When any other object is outside the skin of the hollow sphere, gravity can be computed as if the object is a point mass per usual practice. However, if another object passes through the hole in the skin, all of a sudden the gravitational attraction becomes zero... everywhere inside the hollow sphere. A bit strange, but I can see why that's true.
While testing the physics engine in my 3D game engine, I noticed what appears to be strange behavior when flat triangles come close to each other. In my tests, the triangles have mass, as if they are very thin. In fact they are infinitely thin, because they are simply triangles, but they behave the same when I make them very thin [3-or-more-sided] disks. Sometimes they even explode apart (I think when the centers come close together).
I realized this might be slightly similar to the hollow sphere situation in a way. When the centers of the triangles come closer than the radius of the triangles, the point mass representation seems to have the same characteristic that makes the hollow sphere case change behavior, namely (some physical parts of each object is in opposite directions from the center of gravity (or more precisely, "some physical parts of") the other object. It doesn't feel like exactly the same situation as the hollow sphere, but the behavior is strange and I wonder if that's because this assumption (that point masses are a valid representation) is failing big time in a similar way as the hollow sphere case.
Does anyone know about this stuff and provide some comments, links, references or something?
PS: The rigid body physics and contact routines seem to work and behave properly with thick objects (where the center of masses are not super close to the external surfaces). I don't have code to handle stacking (objects with contacts with multiple other objects), but the behavior I am seeing happens with only two objects.
BTW, on a separate issue, what's the best place to read about going from "simple contacts" like I have (only 1~4 contact points on each of any two objects) to support for "arbitrary stacking"? I'm guessing "stacking" refers to the situation where more than two objects are in contact with each other, with each contact pair having 1~4 points in contact. I say "not more than 4 contacts" simply because my code reduces cases of more-than-4 contacts to 4 contacts to simplify processing.
Thanks in advance for any tips, ideas, references.