Physics Question

I may be expecting / trying to achieve too much with the Physics engine at the moment. I created a very simple level in the level builder with the following kind of structure:

o



-------------------------------
^

Consisting of a static sprite as the pivot (represented by ^), a plank as a static sprite represented by the dotted line and a ball represented by o

I set a constant force on the ball such that it moved down the screen and made the pivot immovable. I was expecting the plank to move / pivot around the pivot when the ball collided with the plank. All had collisions and physics response set to true. But the ball just collided with the plank and remained static.

Am I expecting too much or am I missing something obvious?

--Toby