Attached Link: http://www.bluetones-productions.com/BallBounceOffWood3.gif

With an object that you don't want to introduce until later: Either make a second scene that starts where the new item enters with everything else still in their places, or Make a seperate directional force for each object. In the case of my little scene with the balls: 1- Drag the directional force onto the sphere. Reset it's angle so that it still faces downwards. 2- Duplicate the sphere, (which will duplicate the directional force to.) 3- Set the second directional forces setting for strength. Set it to 0 at frame 1, then go to another frame and set it to the default, (if that's what you want.) It will set a keyframe for that directional force to activate 'gravity' at that time. 4- Play through to see if you like the effect. 5- Render and enjoy! :> The link is to a second animation with the second ball hanging in mid air, the dropping like the first.