Another simple Flash question...

So I'm trying to teach myself basic animation techniques...I made a simple bouncing ball movie (Using the tween: motion command). I want to have it bounce across a floor (which is represented by just a line), but I dont know how to put in the line and have it not interfere with the animation. If I put the line in the first frame, it disappears once the ball's animation starts.

Basically my question is how do I specify which objects in my movie are to be animated from one frame to the next?