flash as3 physics if statement issue :

When velocity is greater than 4 , the ball should stop . i also tried the code   if(xx>4) { ball.stop(); } inside score . and outside scope . . but does not  work

// set gravity amount

var gravity:Number = .000098;

// set starting velcity

var xx:Number = 0.2;

// mark start time and add listener

var lastTime:int = getTimer();

addEventListener(Event.ENTER_FRAME, animateBall);

// step animation

function animateBall(event:Event) {

  // get time difference

  var timeDiff:int = getTimer()-lastTime;

  lastTime += timeDiff;

  // adjust vertical speed for gravity

  xx+= gravity*timeDiff;

  // move ball

  ball.rotation += timeDiff*xx;

}

  if(xx>4) {

  ball.stop();

  }