The Box2D world's inner simulation process advances every frame as the programmer orders in the step function. The specified amount of time will determine the speed of this simulation, having a direct impact on the game experience.
Delimiting a certain number of milliseconds might seem simple but the truth is that there are a good variety of ways to perform this.
The FIX YOUR TIMESTEP! article is the father of this recipe, so do not hesitate to read it at http://gafferongames.com/game-physics/fix-your-timestep/.
Run the code within
Box2DFixedTimeStepSample.java, so you will see a scene where boxes fall from the sky and there are three buttons at the bottom. Each of them enable one of the three approaches covered ...