#### 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

## Density, friction, and restitution

As you already know how to add bodies to the world, I want to introduce you to three attributes that will affect bodies' behavior: density, friction, and restitution.

The density is used to set the mass of a body, measured in kilograms by square meter. Higher density means heavier bodies, and it can't be negative.

The friction comes into play when two bodies slide on each other, and it's defined by a coefficient, usually from 0 (no friction) to 1 (strong friction). It can't be negative.

The restitution determines how much a body will bounce after a collision. Like density and friction, it can't be negative and it's defined by a coefficient usually from 0 to 1. A ball falling on the ground with restitution zero won't bounce (inelastic collision), whereas a value of one would have made the ball bounce with the same velocity it had at the moment of the collision (perfectly elastic collision).

`Density, friction, and restitution must be added to the fixture...`