When the player clicks the mouse, the ball will be launched from the mouse's location. Gravity and the string make the force. The string force is the vector from the launch position to the original position. I set up static properties first, the force is always applied, but it only has value and an effect after the mouse is clicked. 

• How to add a reverse force to the string? 

• How to make the string bounce back and force after launching the ball?