Uh...hit test?

1

i do not understand why my hittest wont work. output panel doesnt give me any errors but my movie clip just sits there. anyone wanna takea look?

onClipEvent (enterFrame) {
    if (_root.ball.hitTest(_root.ground)) {
        _root.gravity *= -1
    }
}
//and i tried this
onClipEvent (enterFrame) {
    if (_root.ball, hitTest(_root.ground)) {
        _root.gravity *= -1
    }
}