Advancing the world time
The box is dynamic but it does not fall down. Are we doing anything wrong here? The answer is no. We have set up the box correctly, but we forget to advance the time in the physics world.
In the Box2D physics world, all calculations are done in a systematic iteration. The world calculates the physical transformation of all things according to the current step. When we move the step
to the next level, the world calculates again as the new state.