ar world = new b2World(
               new b2Vec2(0, 10)    //gravity
            ,  true                 //allow sleep
         );
         
         var fixDef = new b2FixtureDef;
         fixDef.density = 1.0;
         fixDef.friction = 0.5;
         fixDef.restitution = 0.2;
         
         var bodyDef = new b2BodyDef;
         
         //create ground
         bodyDef.type = b2Body.b2_staticBody;
         fixDef.shape = new b2PolygonShape;
         fixDef.shape.SetAsBox(20, 2);
         bodyDef.pos