so, i've updated the project with forces. it works with much less bugs now, but the strange falling through the left side of the bough still appears. you can test it here
it look like there is an invisible hole inside the polygon. just drag the left stone exactly along the bough, to the left side. you will see it is falling down before the end of it.
any ideas to fix this appreciated xeed2012-01-10 14:38:16