adding force

how can i make this script move objects by force, rather than by changing there position?

 private GameObject PullOBJ;
 public float PullSpeed;
 public float objRotationSpeed;
 public float rotation;
 public float PullStrength;
 public void OnTriggerStay (Collider coll)
 {
     if (coll.gameObject.tag == "Untagged") {
         PullOBJ = coll.gameObject;

      PullOBJ.transform.position = Vector3.MoveTowards (PullOBJ.transform.position, this.transform.position, PullSpeed * Time.deltaTime);
      PullOBJ.transform.RotateAround (transform.position, Vector3.up, Time.deltaTime * rotation);
      PullOBJ.transform.Rotate (Vector3.left, 45 * Time.deltaTime * objRotationSpeed);
  }

}