Applying acceleration movieclip

1

I am trying to add acceleration to a movieclip called Ground. I have written this code

package {
import flash.display.Stage;
import flash.display.Sprite;
import flash.ui.;
import flash.events.;

public class Ground extends Sprite{
private var xVel:Number = 0;
private var xAcc:Number = -0.25;

public function Accelerate()
{
    init();
}

private function init():void
{
    stage.addEventListener(KeyboardEvent.KEY_DOWN, go);
}

private function go(event:KeyboardEvent):void
{
    if(event.keyCode == Keyboard.UP){
        xVel += xAcc;
        }
    }
}

}

The movieclip loads fine and i get no errors, but when the up button is pressed nothing happens. Can anyone help?