move ball in X and Z no Y

Physics
1

Hi,
with Unity 2.6.1 trail version, I’m trying re-program “Arkanoid” style game…
move Arkanoid with moving mouse right and left
Left Click: start push the ball
here is my start try:
http://iammfa.webs.com/arkanoid.html
The problem here: you can see the ball jumping a little when moving … I want the ball moves in X and Z directions and disable Y direction, how can I do that?

here is the ball code:

//define some variables
var speed=20;
var colliding : Collider;
var body : Rigidbody;
var forceMultiplier = 25;

//define some objects
var childTransform : Transform;
var parentTransform : Transform;

//define some boolean variables
var stick:boolean=true;
var ballMove:boolean=false;

function FixedUpdate (){

//how many seconds since scene started
var seconds=Time.timeSinceLevelLoad;
//as long as stick variable true, stick ball to arkanoid
if(stick){
childTransform.transform.parent = parentTransform.transform;
}else{
childTransform.transform.parent = null;
}
//is player press space or scene started since 2 seconds
if(Input.GetMouseButton(0) || seconds==2){
stick=false;
ballMove=true;
//ballMove variable be true, move ball to ward top right corner
if(ballMove){
rigidbody.velocity.z=speed;
rigidbody.velocity.x=-speed;
} 
}
}

function OnCollisionEnter (collisionInfo : Collision) {
   if (collisionInfo.gameObject.name == "PinBall") {
      var contactNormal = collisionInfo.contacts[0];
               var rot = Quaternion.FromToRotation(Vector3.up, contactNormal.normal);
               var pos = contactNormal.point;
               var rv = collisionInfo.relativeVelocity.magnitude;
      direction = body.transform.position - pos;
      reflectBallTo = Vector3.Reflect (pos, direction);
      colliding.gameObject.rigidbody.AddForce (reflectBallTo * forceMultiplier);
   }   
}
2 
rigidbody.velocity.y= 0;

You also seem to have a curious mix of scripted movement and rigidbodies. You could easily gut all of the code you have and still have everything work properly.

If you disable gravity and apply frictionless materials, then an add force in the desired direction, it all will work automagically. No need to calculate reflection.

That is after all, why you have a physics engine in the first place.