What is causing the object to fly upwards?

Basically I just want to be able to make the object jump, but whenever I start the game, the ball ends up floating up. I believe the issue lies within either line 20 or 17, but I could very well be mistaken.

 #pragma strict
  
  var rotationSpeed = 100;
 
  var jumpHeight = 8;
 
  private var isFalling = false;
 
  function Update () 
  {
      //Ball rotation.
      var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
      rotation *= Time.deltaTime;
      var rigidbody: Rigidbody = gameObject.GetComponent(Rigidbody) as Rigidbody;
     GetComponent.<Rigidbody> ().AddRelativeTorque (Vector3.back * rotation);
 
     if (Input.GetKeyDown(KeyCode.W) && isFalling == false);
 
     {
         rigidbody.velocity.y = jumpHeight;
     }
     isFalling = true;
 
  }
  function onCollisionStay ()
  {
  isFalling = false;
 
  }