Box2D world is ruled by forces and is able to apply them to bodies to give us a realistic simulation, but if you couldn't manually apply forces to bodies, you won't be able to move bodies, fire bullets, throw birds, drive cars, and do all the awesome things you see when you play physics games.
In this chapter you will learn how to apply forces to move bodies in different ways, managing a lot of new Box2D features including:
Applying a force to a body
Applying an impulse to a body
Setting the linear velocity to a body
Knowing the mass of a body
Applying the right kind of force according to your gameplay needs
Mixing physics with non-physics assets in your game
By the end of the chapter, you will be able to create an Angry Birds level, sling included.