obj为目标坐标
ball为运动物

1.弹簧式运动：
xspeed = 0;
yspeed = 0;
onEnterFrame = function () {
   xspeed = (ball._x-obj._x)*.4+xspeed*.7;
   yspeed = (ball._y-obj._y)*.4+yspeed*.7;
   ball._x -= xspeed;
   ball._y -= yspeed;
};

2.缓冲运动：
onEnterFrame = function () {
ball._x-=(ball._x-obj._x)*.4
ball._y-=(ball._y-obj._y)*.4
｝