Shoot with gravity

hello i have this code for the rotation of a cannon movie clip onClipEvent (enterFrame) {
if (!_root.motionFlag) {

myRadians = Math.atan2(_root._ymouse-this._y, _root._xmouse-this._x);
myDegrees = Math.round((myRadians*180/Math.PI));
_root.yChange = Math.round(_root._ymouse-this._y);
_root.xChange = Math.round(_root._xmouse-this._x);
this._rotation = myDegrees+145;
and i want when i push on it a ball will follow the angle of the cannon but not straight but with gravity can anyone help!!!