Actionscript 3 AS3旋转启动速度，然后随时间减慢

我得到了这个代码，使我的物体不断旋转

movieClip_2.addEventListener(Event.ENTER_FRAME, fl_RotateContinuously_2);

function fl_RotateContinuously_2(event:Event)
{
    movieClip_2.rotation += 100;

}

我怎样才能让这个物体从100秒减速到10秒

var fps:Number = stage.frameRate;
var step:int = 100;
var minStep:int = 10;
var time:Number = 5;
var increment:Number = (step - minStep) / (time * fps);

movieClip_2.addEventListener(Event.ENTER_FRAME, fl_RotateContinuously_2);

function fl_RotateContinuously_2(event:Event)
{
    movieClip_2.rotation += step;
    step -= increment;
    step = Math.max(minStep, step);
}