Throwing a ball, target unrecognized?

Hi,

i am not sure why my example does not work, i would like to throw a ball to a target, but the ball is thrown just behind the "launcher" (the cube) or goes in the air, but never goes toward my target. Would you know why? Here is an image (the target is on the left), so the ball seems to take a random value each time it is thrown :

and the code :

 var target : Transform;
 var cannonball : GameObject;
 
 function Update () {
     if (Input.GetKeyDown("b")){  // press b to shoot
         var ball: GameObject = Instantiate(cannonball, transform.position, Quaternion.identity);
         var velocity = rigidbody.velocity;
         var targetVelocity = (target.position - transform.position).normalized;
             targetVelocity = targetVelocity * 25.0;
         var velocityChange = targetVelocity - velocity;
         //velocityChange.y = 0;
         //Vector3.forward*4, ForceMode.Impulse
         ball.rigidbody.AddForce(velocityChange, ForceMode.Force);
         Destroy(ball, 10);
     }
 }

At the end i would like to throw the ball to a specific target and with an angle, like in this thread : http://answers.unity3d.com/questions/148399/shooting-a-cannonball.html

Thanks for your help