Hi guys, I really don't know how the ball that I control can roll in Z Axis can you help me please? Thank you very much :D

pragma strict

 var rotationSpeed = 100;
 var jumpHeight = 8;
 
 private var isFalling = false;
 
 function Update ()
 {
     //Handle ball rotation.
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
     rotation *= Time.deltaTime;
     rigidbody.AddRelativeTorque (Vector3.back * rotation);
     
     if (Input.GetKeyDown(KeyCode.Space) && isFalling == false)
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
     
 
 }
 
 function OnCollisionStay ()
 {
     isFalling = false;
 }