slide an object by using mouse.

when i try the game and left click the mouse, it only come out a dot, not forming a complete slice and the dot is falling down, the y axis keep decreasing.

example game: fruit ninja.

 public GameObject bladeTrailPrefab;
     public float minCuttingVelocity = .001f;
     bool isCutting = false;
     Vector2 previousPosition;
     GameObject currentBladeTrail;
     Rigidbody2D rb;
     Camera cam;
     CircleCollider2D circleCollider;
 
     void Start ()
     {
         cam = Camera.main;
         rb = GetComponent<Rigidbody2D>();
         circleCollider = GetComponent<CircleCollider2D>();
     }
 
     // Update is called once per frame
     void Update () {
         if (Input.GetMouseButtonDown(0))
         {
             StartCutting();
         } else if (Input.GetMouseButtonUp(0))
         {
             StopCutting();
         }
 
         if (isCutting)
         {
             UpdateCut();
         }
 
     }
 
     void UpdateCut ()
     {
         Vector2 newPosition = cam.ScreenToWorldPoint(Input.mousePosition);
         rb.position = newPosition;
 
         float velocity = (newPosition - previousPosition).magnitude * Time.deltaTime;
         if (velocity < minCuttingVelocity)
         {
             circleCollider.enabled = true;
         } else
         {
             circleCollider.enabled = false;
         }
 
         previousPosition = newPosition;
     }
 
     void StartCutting ()
     {
         isCutting = true;
         currentBladeTrail = Instantiate(bladeTrailPrefab, transform);
         previousPosition = cam.ScreenToWorldPoint(Input.mousePosition);
         circleCollider.enabled = false;
     }
 
     void StopCutting ()
     {
         isCutting = false;
         currentBladeTrail.transform.SetParent(null);
         Destroy(currentBladeTrail, 2f);
         circleCollider.enabled = false;
     }