Made gravity NOW I've got to make collision

Hi I’ve been working on a gravity effect to effect a duplicated “ball” movieclip on an FLA which I’ve now done.

The next step is to get all the duped balls to “stack” on top of eachother using a collision method. I’m not sure how this can be done with the way I have my FLA…Here is how I have it layed out (see attachment).

And that’s it!

What would I need to do to start getting them to “stack”?

Any help would be appreciated