3D object with a 2D child?

So I made a simple segmented tall box with bones in Maya and imported to Unity to test it as a rope. This "rope" acts like a rope when I play the scene. I have it hanging, I move the lower bone, it swings, etc,

I attached an empty 2d game object to the last bone as a child, and then I hinged a sprite with collider to that 2d game object.

So when I play, if I swing that last bone the sprite swings with it. If I collide with the sprite with a 2d charater, it moves the sprite but doesnt drag the 3d rope with it. Would anyone be able to explain why?

 

2D physics and 3D physics are separate engines and don't interact.

--Eric

 

Yes, that I understand, but Im trying to make a 2d object interact with a 2d object that is the child of a 3d object. If I swing the 3d object it swings the child 2d object with it.

Ive read other people mention this a while back, but Ive never seen it operational. So I was wondering if this 2d child to a 3d object is something valid. And if so then why does moving the child 2d object not move the 3d object, but if I move the 3d object it DOES move the 2d child with it.