Final answer:
The direction of the drag force on a tennis ball going straight up and down changes with its motion: downwards when ascending and upwards when descending.
Step-by-step explanation:
When a tennis ball is hit straight up and then comes back down, the drag force always opposes the direction of the tennis ball's velocity. Therefore, while the ball is moving upward, the drag force is directed downwards. Conversely, when the ball is falling back down, the drag force is directed upwards. It is important to note that the direction of drag force changes with the direction of the ball’s motion relative to the air through which it moves.