Moving a movieclip towards the mouse but stopping short

I'm experimenting with different control schemes for something I'm working on at the mo开发者_开发知识库ment and I've currently made it so that a movieclip can follow my mouse cursor. My problem is that it comes and sits right on the point of the cursor (as it's 'supposed' to). I'd like to have a radius of around 10px surrounding the cursor that the movieclip would stop at so that it heads towards the cursor but stops short. The only way I can think to do this would be with collision detection but that seems like a very clumsy route to take.

Here's my code that moves the movieclip towards the mouse:

this is run on ENTER_FRAME

var dx:Number = stage.mouseX - this.x;
var dy:Number = stage.mouseY - this.y;
this._vx = Math.cos(Math.atan2(dy, dx)) * this._speed;
this._vy = Math.sin(Math.atan2(dy, dx)) * this._speed;
this.x += this._vx;
this.y += this._vy;

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Why not use Pythagoras theorem to check the distance?

var a:Number = stage.mouseX - x;
var b:Number = stage.mouseY - y;

var dist:Number = Math.sqrt(a*a + b*b);
if(dist > 10)
{
    _vx = Math.cos(Math.atan2(b, a)) * _speed;
    _vy = Math.sin(Math.atan2(b, a)) * _speed;
    x += _vx;
    y += _vy;
}

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package { import com.Ball; import flash.display.Sprite; import flash.events.Event;

public class Main extends Sprite {
    private var bal:Ball;
    private var easing:Number=.3;

    public function Main():void {
        init();
    }
    private function init():void {
        bal=new Ball(8,Math.random() * 0xffffff);
        addChild(bal);
        addEventListener(Event.ENTER_FRAME,animAction);
    }
    private function animAction(e:Event):void {
        var dx:Number=mouseX - bal.x;
        var dy:Number=mouseY - bal.y;
        var ax:Number=dx * easing;
        var ay:Number=dy * easing;
        bal.x+= ax;
        bal.y+= ay;
    }
}

}