#### Overview of this book

Physics games are getting more and more popular, and Box2D is the best choice if you are looking for a free, stable and robust library to handle physics. With Box2D you can create every kind of 2D physics game, only coding is not the fun part, but the game itself. "Box2D for Flash Games" will guide you through the process of making a Flash physics game starting from the bare bones and taking you by hand through complex features such as forces, joints and motors. As you are learning, your game will have more and more features, like the physics games you are used to playing. The book analyzes two of the most played physics games, and breaks them down to allow readers to build them from scratch in a step-by-step approach. By the end of the book, you will learn how to create basic primitive bodies as well as complex, compound bodies. Motors will give life to cars, catapults and siege machines firing bullets, while a complete collision management will make your game look even more realistic. If you want to make full Flash games with physics, then Box2D for Flash Games will guide you through the entire process of making a Flash physics game.
Box2D for Flash Games
Credits
www.PacktPub.com
Preface
Free Chapter
Hello Box2D World
Interacting with Bodies
Applying Forces to Bodies
Skinning the Game
Bullets and Sensors
Index

## Creating a circular shape

A shape is a 2D geometrical object, such as a circle or a polygon, which in this case must be convex (every internal angle must be less than 180 degrees). Remember, Box2D can only handle convex shapes.

At the moment, we are starting with the ball, so we'll create the circle:

```var circleShape:b2CircleShape;
circleShape=new b2CircleShape(25/worldScale);```

`b2CircleShape` is used to create a circular shape, and its constructor wants the radius as an argument. With the previous lines, we are creating a circle whose radius is `25` pixels, thanks to our friend—the `worldScale` variable. From now on, every time you want to work with pixels, you'll have to divide them by `worldScale`. You can also define a function called, let's say, `pixelsToMeters` , and call it every time you need to convert pixels to meters.

When we have a body definition and a shape, we can glue them together using a fixture.