package
{
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.events.KeyboardEvent;
    import flash.events.MouseEvent;
    import flash.ui.Keyboard;

import net.hires.debug.Stats;

[SWF(frameRate=30, width=465, height=465, backgroundColor=0xffffff)]

public class Main extends Sprite
    {
        public static const NUM_HANDLES:int = 4;
        public static const SPRING:Number = 0.15;
        public static const FRICTION:Number = 0.8;

private var handles:Vector.<Ball>;
        private var ball:Ball;
        private var stats:Stats;

/**
         *  コンストラクタ
         */
        public function Main()
        {
            addEventListener(Event.ADDED_TO_STAGE, initialize);
        }

/**
         *  初期化
         */
        private function initialize(evt:Event):void
        {
            removeEventListener(Event.ADDED_TO_STAGE, initialize);

var i:int,
                b:Ball;

handles = new Vector.<Ball>(NUM_HANDLES, true);
            for (i=0; i<NUM_HANDLES; i++)
            {
                b = new Ball(5, 0xa8a8a8);
                b.x = Math.random() * stage.stageWidth;
                b.y = Math.random() * stage.stageHeight;
                b.buttonMode = true;
                b.addEventListener(MouseEvent.MOUSE_DOWN, dragged);

handles[i] = b;
                addChild(b);
            }

ball = new Ball(20, 0xff3c3c);
            addChild(ball);

//  for debug.
            stats = new Stats();
            stats.visible = false;
            addChild(stats);

addEventListener(MouseEvent.MOUSE_UP, dropped);
            addEventListener(Event.ENTER_FRAME, step);
            stage.addEventListener(KeyboardEvent.KEY_DOWN, toggle);
        }

/**
         *  毎フレームの処理
         */
        private function step(evt:Event):void
        {
            var i:int,
                b:Ball,
                dx:Number,
                dy:Number;

for (i=0; i<NUM_HANDLES; i++)
            {
                b = handles[i] as Ball;

dx = b.x - ball.x;
                dy = b.y - ball.y;

ball.vx += dx * SPRING;
                ball.vy += dy * SPRING;
            }

ball.vx *= FRICTION;
            ball.vy *= FRICTION;
            ball.x += ball.vx;
            ball.y += ball.vy;

graphics.clear();
            graphics.lineStyle(1, 0xa8a8a8);
            for (i=0; i<NUM_HANDLES; i++)
            {
                b = handles[i] as Ball;

graphics.moveTo(ball.x, ball.y);
                graphics.lineTo(b.x, b.y);
            }
        }

/**
         *  ドラッグ時の処理
         */
        private function dragged(evt:MouseEvent):void
        {
            evt.target.startDrag();
        }

/**
         *  ドロップ時の処理
         */
        private function dropped(evt:MouseEvent):void
        {
            stopDrag();
        }

/**
         *  キータイプ時の処理
         *  ctrl + space で Stats の表示を切り替え
         */
        private function toggle(evt:KeyboardEvent):void
        {
            if (evt.keyCode == Keyboard.SPACE && evt.ctrlKey == true)
            {
                stats.visible = !stats.visible;            
            }
        }
    }
}

import flash.display.Sprite;

class Ball extends Sprite
{
    public var vx:Number;
    public var vy:Number;

public function Ball(radius:Number=3, color:uint=0)
    {
        vx = 0;
        vy = 0;

graphics.clear();
        graphics.beginFill(color);
        graphics.drawCircle(0, 0, radius);
        graphics.endFill();
    }
}

バネの復習とか