Shooting a cannonball

Hi, i want to do a game like peggle. I have a rotative cannon and a ball. The ball lauch but don't lauch in the direction of the cannon. The ball just drop down. I would like that the ball lauch in the direction the cannon is pointing. Here is what i have for now:

using UnityEngine;

using System.Collections;

public class lancer : MonoBehaviour {

 public Transform bouton;
 public Transform balle;
 public bool lance = false;
 public Rigidbody projectile;
 
 void OnMouseDown()
 {
 if (lance == false)
     {
     projectile.rigidbody.AddForce(Vector3.forward * 10, ForceMode.Impulse);
     balle.rigidbody.useGravity = true;
     lance = true;
     }
 }

}