private function checkCollision(ball0:animation_ch11_Ball, ball1:animation_ch11_Ball):void
    {
     var dx:Number = ball1.x - ball0.x;
     var dy:Number = ball1.y - ball0.y;
     var dist:Number = Math.sqrt(dx*dx + dy*dy);
     if(dist < ball0.radius + ball1.radius)
     {
      // calculate angle, sine and cosine
      var angle:Number = Math.atan2(dy, dx);
      var sin:Number = Math.sin(angle);
      var cos:Number = Math.cos(angle);
      
      // rotate ball0's position
      var pos0:Point = new Point(0, 0);
      
      // rotate ball1's position
      var pos1:Point = rotate(dx, dy, sin, cos, true);
      
      // rotate ball0's velocity
      var vel0:Point = rotate(ball0.vx,
            ball0.vy,
            sin,
            cos,
            true);
      
      // rotate ball1's velocity
      var vel1:Point = rotate(ball1.vx,
            ball1.vy,
            sin,
            cos,
            true);