Hi again!

now i found another problem!!! i searched for it but i can't find any thing that i can understant it! by the way, i want to write stack box game, and i nead to create a new box when last box reaches on floor? i don't know how can define dynamic object?
also i don't have any idea about this:
"A box stops moving when it has been placed on top of the top-most resting box, where “on top” means that the barycenter of the bottom face of the falling box touches the top face of the top-most resting box"
because for solibox just i set size not any thing else! so how can i recognize barycenteric of the one face?!