this.car = new Box(x, y, w, h, {

    restitution: 0

  });

  this.leftWheel = new Ball(x - w / 2 + r, y + h / 2 - r / 2, r, {

    restitution: 0

  });

  this.rightWheel = new Ball(x + w / 2 - r, y + h / 2 - r / 2, r, {

    restitution: 0

  });

  World.add(world, [this.car, this.leftWheel, this.rightWheel]);

​

  this.show = function() {

    this.car.show();

    this.leftWheel.show();

    this.rightWheel.show();

  }

}