Проблема с отскоком объекта

[Hauts]

var stageWidth = 1024;
var stageHeight = 768;
var speed = 10;
 
var ball = createBall();
 
var randomStartAngle = Math.random() * Math.PI * 2;
ball.vx = Math.cos(randomStartAngle);
ball.vy = Math.sin(randomStartAngle);
ball._x = stageWidth / 2;
ball._y = stageHeight / 2;
 
function createBall() {
	var d = this.getNextHighestDepth();
	var ballInstance = this.createEmptyMovieClip("ball_" + d, d);
	ballInstance.beginFill(0x000000,100);
	ballInstance.moveTo(-10,-10);
	ballInstance.lineTo(10,-10);
	ballInstance.lineTo(10,10);
	ballInstance.lineTo(-10,10);
	ballInstance.lineTo(-10,-10);
	ballInstance.endFill();
	return ballInstance;
}
this.onEnterFrame = function() {
	var newPosX = ball._x + ball.vx * speed;
	var newPosY = ball._y + ball.vx * speed;
	if (newPosX < 0 || newPosX > stageWidth) {
		ball.vx *= -1;
	}
	if (newPosY < 0 || newPosY > stageHeight) {
		ball.vy *= -1;
	}
	ball._x += ball.vx * speed;
	ball._y += ball.vy * speed;
};