Locking an object's pivot point in game engine In the game engine, how do you lock an object's translation while letting it freely rotate?

I've got a record with grooves, with random noise added to all vertices in the z direction.



The needle/arm should drop into a groove on the record, and be free to rotate about the end of the arm as the grove moves up and down.

If I lock the x/y/z translations of the arm, it doesn't move at all. I don't understand why this doesn't simply lock the pivot point, and give me the desired affect.
If I understand the question correctly, the needle is holding the arm up, as it cannot move, only rotate. (locking the translations does let it rotate, but it gets held whereever it starts.)



If that is the case, then you need Rigid Body Joint constraints.
Then the needle will be able to move, but only in relation to the arm.

Here is a quick setup: