rolling ball movement on different axis

hey. Am making a ball rolling game where you go down a hill destroying things and have the ball moving but not how i want. the JS is

var xForce : float = 10; var yForce : float = 10; var zForce : float = 10;

function Update () {

if (Input.GetKey ("up")) rigidbody.AddForce(Vector3(0, 0, 10));

 if (Input.GetKey ("right"))
     rigidbody.AddForce(Vector3(10, 0, 0));

 if (Input.GetKey ("down"))
     rigidbody.AddForce(-Vector3(0, 0, 10));

 if (Input.GetKey ("left"))
     rigidbody.AddForce(-Vector3(10, 0, 0));

}

as you can see the ball moves up, left, down and right but because the level/terrain is a spiral shape, as you are going down the directional buttons will change the way the ball is moving because the directions are related to the terrain and not the ball. makes sense? so can someone please help me make the ball travel forward when up is pressed etc all the way down a spiral level please?