Instantiating a prefab with velocity

I am pretty new to Unity and am trying to make an Infinite runner game. I have the floor generating randomly, however I cannot get the floor to move at all, I have tried using both addForce and adding a velocity to the rigidbody, bot neither have worked.

 public Transform mainPrefab;
 public Rigidbody floorSpeed;
 public Vector3 startPosition;
 public Vector3 nextPosition;
 public int numberOfObjects;

 private Queue<Transform> objectQueue;

 void Start    () {
     objectQueue = new Queue<Transform>(numberOfObjects);
     nextPosition = startPosition;
     for (int i = 0; i < numberOfObjects; i++){
         Transform o = (Transform)Instantiate(mainPrefab);
         o.localPosition = nextPosition;
         nextPosition.z += o.localScale.z*5;
         objectQueue.Enqueue (o);
         floorSpeed = o.GetComponent<Rigidbody>();
         floorSpeed.AddForce(0, 0, -1);
     }
 }