private function init(e:Event = null):void
{
    removeEventListener(Event.ADDED_TO_STAGE, init);
    // entry point
    addChild(s);


    var bounding:b2AABB = new b2AABB();
    bounding.lowerBound.Set( - 100, - 100);
    bounding.upperBound.Set( 1000, 1000);
    w  = new b2World(bounding, new b2Vec2(0, 100), true);
    var groundBodyDef:b2BodyDef = new b2BodyDef();
    groundBodyDef.position.Set(400, 580);
    var groundBody:b2Body = w.CreateBody(groundBodyDef);
    var groundShapeDef:b2PolygonDef = new b2PolygonDef();
    groundShapeDef.SetAsBox(400, 20);
    groundBody.CreateShape(groundShapeDef);
    //
    var circleBodyDef:b2BodyDef = new b2BodyDef();
    circleBodyDef.position.Set(300, 200);
    var circleBody:b2Body = w.CreateBody(circleBodyDef);
    circleBody.m_type = b2Body.e_dynamicType;
    var circleDef:b2CircleDef = new b2CircleDef();
    circleDef.radius = 10;
    circleDef.density = 2;
    circleDef.friction = 0.5;
    circleDef.restitution = 0.5;
    circleBody.CreateShape(circleDef);
    circleBody.SetMassFromShapes();
    addEventListener(Event.ENTER_FRAME, update);
}