Gravity

I just started using TGB today. I am trying to figure out how to get a ball to fall and hit a platform. I followed the Collisions Tutorial that is in the documentation. I get the ball to fall, but it does not fall with gravitational acceleration. I mean it just falls at the speed I set and that is it. How do I get it to fall with acceleration. Also the ball goes through the platform. Not a first, as it is coming to a rest the bounces around a lot, and starts to sink through the floor. How do I fix this?

Thanks