Tennis racquet string tension and ball speed
The hypothesis that the racquet with the lower string tension will make the ball travel faster, is proven to be true.
The speed of the ball is determined by the combination of racquet speed and power. When the ball is hit near the neck of the racquet, more power is gained but te speed is lost. Hitting the ball near the tip of the racquet will provide greater speed but it will lack in power. It is therefore up to the player to find the best spot at which to hit the ball, such that it complements the player's strength, speed and skill.
This science fair project can also be conducted using racquets of different weights.
Perhaps you could also measure the distance traveled by the ball?
Why is maximum power in the throat and the maximum shot speed toward the tip? - http://twu.tennis-warehouse.com/learning_center/PPandspeed.php
Power: Does the racquet matter? - http://twu.tennis-warehouse.com/learning_center/racquetcontribution.php
Basic safety requirements