Making bubbles rise up while heavy objects fall down? Please help!

I’m trying to write a simple water game where I use bubbles released from the bottom of the screen to keep heavier objects from falling to the bottom. The only way I can get the bubbles to float up is to change the direction of gravity… unfortunately I still need the heavier objects to fall down. Is there a way to define gravity differently for different object or is there some sort of buoyancy setting?
Thanks so much!
Tim [import]uid: 16901 topic_id: 6073 reply_id: 306073[/import]

Watch the 8 minutes video on the video page.

use force(x, -y) [import]uid: 6587 topic_id: 6073 reply_id: 20796[/import]

Thanks! [import]uid: 16901 topic_id: 6073 reply_id: 20797[/import]