firstly i drew a simple shape and made it move up and down using the motion tween and keyframes.

Here is the first key frame.
The shape was drawn using the paint brush, and then the frame turned into a keyframe by pressing I. i then extended the exposure by 10 frames. this copies the same image across the next 10 frames. the middle and last frames were also changed to keyframes.

Here is the middle frame. i moved the drawing downwards using the motion arrow. now when the animation is played the middle keyframe is different to the first and last frames, creating motion. the other frames are automatically filled in with motion by the computer.