1 Open new doc, set as 24fps. Draw a ground line on the first layer, rename it to ‘BG’, and convert the line to a symbol (movie clip)
2 Create a ball (hold shift to keep it circular). You can delete the outer line and just leave the fill. Convert the ball to a symbol. Double click the ball to enter symbol editing mode, then use the transform tool to move the registration point to the bottom of the ball.
3. Move the ball off screen, then insert a keyframe at frame 12. On frame 12, move the ball vertically downward to connect with the ground. at frame 4, insert another keyframe and move the ball slightly down. go to frame 7, insert keyframe, then move it further down (with a bigger gap between 4 and 7 than 1 and 4, to count for acceleration
4. Between each keyframe insert a motion tween.
5. Go to frame 16 and insert a keyframe, with the ball still on the ground. Transform the ball so that it is shorter and a little wider than it was, to animate the ball squashing. Remember to insert a motion tween.
6. A quick way of repeating the animation is to select all frames, go to Edit > Timeline > Copy frames, then Edit > Timeline > Paste Frames.