onEnterFrame script moving issue!

Hello!

I use this code on a mc to make it move to a y possition:

Code:

---

onClipEvent(load)
{
_root.ypos1=Stage.height/2;
}
onClipEvent(enterFrame)
{
this._y+=(_root.ypos1-this._y)/3;
}

---

To make it move to y = 20 i use this code on a button:

Code:

---

on(press){
_root.ypos1=20;
}

---

As you can see the movment will go from a fast speed and then slow down.

My question is; Can you make the movement go from 0 then increase and then "break" and stop when it has reached "ypos1=20"?

Thanks in advance!