摩擦力：
假如一个物体在某个方向上沿直线运行，摩擦力会使该方向上的速度越来越小，直到停止。
上图示意了该过程，物体以moveAngle角度正向运动，最终的速度speed矢量为vx矢量与vy矢量的矢量和，在每个单位时间内的位移即Speed矢量的大小，分解到x,y轴后，即为vx与vy；加入摩擦力后，speed矢量每单位时间将减少Friction值，也就是视觉上的越来越慢。
var ball:Ball = new Ball(10);
ball.x = stage.stageWidth/2;
ball.y = stage.stageHeight/2;
addChild(ball);
Mouse.cursor = MouseCursor.BUTTON;
var Velocity:Number = 10;//速度最大值
var friction = 0.4;//摩擦力因子
stage.addEventListener(MouseEvent.MOUSE_DOWN,MouseDownHandler);
stage.addEventListener(MouseEvent.MOUSE_UP,MouseUpHandler);function MouseDownHandler(e:MouseEvent):void{graphics.clear();//初始化小球位置以速度ball.x = stage.stageWidth/2;ball.y = stage.stageHeight/2;ball.vx = (Math.random()*2-1) * Velocity;ball.vy = (Math.random()*2-1) * Velocity; graphics.moveTo(ball.x,ball.y);graphics.lineStyle(1,0xcccccc,1);
}function MouseUpHandler(e:MouseEvent):void{ addEventListener(Event.ENTER_FRAME,EnterFrameHandler);
}function EnterFrameHandler(e:Event):void{ball.x += ball.vx;ball.y += ball.vy;var speed:Number = Math.sqrt(ball.vx*ball.vx + ball.vy*ball.vy);var moveAngle = Math.atan2(ball.vy,ball.vx);speed -= friction;//减速后的新速度ball.vx = speed*Math.cos(moveAngle);ball.vy = speed*Math.sin(moveAngle);//防止减速过度，就成反向运动if (speed<=friction){ball.vx = 0;ball.vy = 0;removeEventListener(Event.ENTER_FRAME,EnterFrameHandler);} graphics.lineTo(ball.x,ball.y);}
上面这种方法从物理意义上讲最接近现实情况，不过有些复杂，在实际开发中还有一种更简单的办法，虽然不怎么严密，但从视觉效果上很难看出问题
var ball:Ball = new Ball(10);
ball.x = stage.stageWidth/2;
ball.y = stage.stageHeight/2;
addChild(ball);
Mouse.cursor = MouseCursor.BUTTON;
var Velocity:Number = 10;
var friction = 0.9;//摩擦力因子(小于1大于0即可)
stage.addEventListener(MouseEvent.MOUSE_DOWN,MouseDownHandler);
stage.addEventListener(MouseEvent.MOUSE_UP,MouseUpHandler);function MouseDownHandler(e:MouseEvent):void{graphics.clear(); ball.x = stage.stageWidth/2;ball.y = stage.stageHeight/2;ball.vx = (Math.random()*2-1) * Velocity;ball.vy = (Math.random()*2-1) * Velocity; graphics.moveTo(ball.x,ball.y);graphics.lineStyle(1,0xcccccc,1);
}function MouseUpHandler(e:MouseEvent):void{ addEventListener(Event.ENTER_FRAME,EnterFrameHandler);
}function EnterFrameHandler(e:Event):void{ball.x += ball.vx;ball.y += ball.vy;ball.vx = ball.vx * friction;//直接让x轴速度不断衰减ball.vy = ball.vy * friction;//直接让y轴速度不断衰减 if (Math.abs(ball.vx)<=0.0001 || Math.abs(ball.vy)<=0.0001){ball.vx = 0;ball.vy = 0;removeEventListener(Event.ENTER_FRAME,EnterFrameHandler);} //trace(ball.vx);//trace(ball.vy);graphics.lineTo(ball.x,ball.y);}
屏幕环绕：
这个叫法也许从字面上不太直观，说得更白一点就是：一个物体如果在运动过程中跑出了舞台边界，开发人员就要想办法让其从舞台的另一端出现，并继续运动，以保持连贯。前面的一篇文章 Flash/Flex学习笔记(23)：运动学原理 中有一个飞船的示例，加入屏幕环绕处理后，代码如下：
package {import flash.display.Sprite;import flash.events.Event;import flash.events.KeyboardEvent;import flash.ui.Keyboard;import flash.display.StageAlign;import flash.display.StageScaleMode;import fl.controls.Label;public class ShipSim2 extends Sprite {private var ship:Ship;private var vr:Number=0;private var thrust:Number=0;private var vx:Number=0;private var vy:Number=0; public function ShipSim2() {init();}private function init():void {stage.scaleMode=StageScaleMode.NO_SCALE;stage.align=StageAlign.TOP_LEFT; ship = new Ship();addChild(ship);ship.x=stage.stageWidth/2;ship.y=stage.stageHeight/2;addEventListener(Event.ENTER_FRAME, EnterFrameHandler);stage.addEventListener(KeyboardEvent.KEY_DOWN, KeyDownHandler);stage.addEventListener(KeyboardEvent.KEY_UP, KeyUpHandler);}private function KeyDownHandler(event:KeyboardEvent):void {switch (event.keyCode) {case Keyboard.LEFT :vr=-5;break;case Keyboard.RIGHT :vr=5;break;case Keyboard.UP :thrust=0.2;ship.draw(true);break;default :break;}}private function KeyUpHandler(event:KeyboardEvent):void {vr=0; thrust=0;ship.draw(false);}private function EnterFrameHandler(event:Event):void {ship.rotation+=vr;var angle:Number=ship.rotation*Math.PI/180;var ax:Number=Math.cos(angle)*thrust;var ay:Number=Math.sin(angle)*thrust;vx+=ax;vy+=ay;ship.x+=vx;ship.y+=vy;var left:Number=0;var right:Number=stage.stageWidth;var top:Number=0;var bottom:Number=stage.stageHeight;//屏幕环绕处理if (ship.x>right + ship.width/2) {ship.x=left-ship.width/2;} else if (ship.x < left - ship.width/2) {ship.x=right+ship.width/2;}if (ship.y-ship.height/2>bottom) {ship.y=top-ship.height/2;} else if (ship.y < top - ship.height / 2) {ship.y=bottom+ship.height/2;}}}
}
最后把这二者结合起来，看下效果：