Create a new FLA file in Flash Professional CS5 and write the following frame action:

var test:World = new World();

[Test Movie] generates the compile error #1136:

Incorrect number of arguments. Expected 2.

I have never defined the class World but tried to solve the errors by adding several functions as follows:

var test:World = new World(this, this);
function createWorld() {}
function getStepSize(temp) {}
function setStepSize(temp, temp2) {}
trace(test);  // Output: [object World]

Both Flash CS4 porfessional and Flash Professional CS5.5 don’t have such class, “World”. I suspect Adobe left the unnecessary class in CS5 by mistake. You have to be careful if you define the class named “World” in CS5.

[postscript: 2011/09/23]
According to the comments below and the blog article “Flash CS5 Built-in Physics Discovered“, the class World is a part of physics engine in the inverse Inverse Kinematics framework. And once the API was planned to release with the “Physics” panel but was abandoned by Adobe.

Based on the blog the following code was written to test the library:

const COUNT:uint = 50;
var instances:Vector.<Shape> = drawShapes(COUNT);
var physics:PhysicsManager = new PhysicsManager(stage);
var myWorld:World = physics.createWorld();
assignPhysics();
myWorld.enableCollisions(true);
myWorld.setGravity(new Point(0, 200));
stage.addEventListener(MouseEvent.CLICK, startPhysics);
function startPhysics(eventObject:MouseEvent):void {
	addEventListener(Event.ENTER_FRAME, animatePhysics);
}
function assignPhysics():void {
	var myPoint:Point = new Point(1, 1);
	for (var i:uint = 0; i < COUNT; i++) {
		var instance:DisplayObject = instances[i];
		var physicsObject:PhysObj = myWorld.addPhysObj(instance, myPoint, 0, false);
		if (i % 2) {
			physicsObject.setNonMoving(true);
		}
	}
}
function animatePhysics(eventObject:Event):void {
	myWorld.updateAllObjects();
	myWorld.step();
}
function drawShapes(count:uint):Vector.<Shape> {
	var nStageWidth:int = stage.stageWidth;
	var nStageHeight:int = stage.stageHeight;
	var nWidth:Number = 25;
	var nHeight:Number = 5;
	var instances:Vector.<Shape> = new Vector.<Shape>(count);
	for (var i:uint = 0; i < count; i++) {
		var myShape:Shape = new Shape();
		var myGraphics:Graphics = myShape.graphics;
		myGraphics.beginFill(uint(0xFFFFFF * Math.random()));
		myGraphics.drawRect(-nWidth / 2, -nHeight / 2, nWidth, nHeight);
		myGraphics.endFill();
		addChild(myShape);
		instances[i] = myShape;
		myShape.x = nStageWidth * Math.random();
		myShape.y = nStageHeight * Math.random();
		myShape.rotation = 360 * Math.random();
	}
	return instances;
}

Just copy the whole code and paste it into the first frame of the main timeline in Flash Professional CS5. Then do [Test Movie] and click on the stage. A half of instances randomly placed will fall down.

Instances are randomly placed on the stage

Instances are randomly placed on the stage

Click on the stage and a half of Instances fall down

Click on the stage and a half of Instances fall down

Flash Professional CS5 includes the PffLib.swc library file in the application folder

Flash Professional CS5 includes the PffLib.swc library file in the application holder

The API of the physics engine is stored in the application folder of Flash CS5, Adobe Flash CS5/Common/Configuration/ActionScript 3.0/libs. You may use the file in CS5.5 as well by adding its folder path to the [Library path] in the [Advanced ActionScript 3.0 Settings].