# rigid body interacting strangely with collision plane -- jittering and fall-through

http://pasteall.org/blend/index.php?id=44844

I am puzzled by the behaviour of the rectangular rigid-body slab in this blendfile. When the wrecking ball hits it, it spins and falls in a reasonable way; but when it hits the floor, it tries to fall through (even though it previously respected the solidity of the floor) and then jitters/jiggles very strangely. It is of a reasonable size (i.e. not tiny) and otherwise a pretty boring object. At first I thought it might have too few vertices so I subdivided it (W) a couple of times but this did not fix the weirdness. Is this a recognised problem with a well-known workaround or solution?

Try applying the scale of all your rigidbodies (⎈ CtrlA), the cube and the floor in particular:

In addition, here are some general practices I like to adhere to when creating rigidbody objects:

• Always apply scale (⎈ CtrlA > Scale).
I don't really know why this matters; one would think the simulator would operate on the scaled mesh and not even be aware of the concept of object scale at all, but apparently not.
• Center the origin to center of mass (⎈ Ctrl⇧ Shift⎇ AltC > Center of mass)
You may have noticed that your wreaking ball was swinging in a slightly strange manner. The reason for this was that the object origin (and thus the center of mass) was located up in the "handle" bit. This simulated as if the handle was much denser (thus heavier) than the ball, which isn't quite the natural expectation ;)
• Use meshes with thickness
Not strictly speaking required, but I have personally found it helps reduce the likelihood of small high-velocity objects failing to collide, particularly with big things like ground planes.
• Avoid mesh collision shapes where possible
I've found collisions involving mesh collision shapes by far the most prone to wonky behavior. Use mesh only when it's absolutely needed; favor the more stable convex hull or, if applicable, the appropriate primitive shapes.
• Thank you gandalf3, I am indeed learning that Mesh collision shapes are not to be trusted :-) I had some bowling pins lined up to be knocked down by the wrecking ball, and they started sinking into the ground plane then springing into the air like jack in the boxes... I could not get them under control until I turned off Mesh and turned on Convex Hull. I did not know about the importance of Apply Scale, many thanks (and for spotting my origin error). Dec 29, 2016 at 7:00