Is there a major problem in ActionScript or am I being an idiot?

[marinebro0306]

I was experimeting with gravity in KM, and I cannot make a ball fall faster and faster using ActionScript.

My code:
onClipEvent(load){
speedX = 5;}

onClipEvent(enterFrame){
this._x += speedX;}

The code is inside a MC of a ball.

Any ideas???
From what I can see, nothing is wrong.