Hi Ram, I am not familiar with the hello world project. Chances are you need to use a physics sprite that knows how to interpret itself in the box2d world, so you can transform it according to the box2d solvers.
For instance, you know that you can put a sprite on the screen and make it rotate and translate on the screen. Now you create the box2d world, create a rectangle for your sprite, give it physical properties (the rectangle), and make it do something in box2d by assigning a force to it. Nothing happens. Boo.
So then you setup the box2d query callbacks so you are told when things move. Then you use those callbacks to adjust the properties of your sprite that is assigned to the rectangle (b2Body). The smarter way is to assign the b2Body to your sprite (a physics sprite now), and let the sprite intercept callbacks from box2d so that it manages itself. viola! now your sprite moves with the rectangle in box2d.
Put another sprite in there with an opposite force applied, and watch them collide!