Help me please... I have this script but its doesn't drag objects

 using UnityEngine; 
 using System.Collections;
 
 public class MouseManager : MonoBehaviour {
 
     Rigidbody2D grabbedObject = null;
     
     Vector2 mousePos2D;
 
     float dragSpeed = 4f;
 
     void Update() {
         if (Input.GetMouseButtonDown (0)) {
             Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint (Input.mousePosition);
             mousePos2D = new Vector2 (mouseWorldPos3D.x, mouseWorldPos3D.y);
         }
         
         Vector2 dir = Vector2.zero;
         RaycastHit2D hit = Physics2D.Raycast (mousePos2D, dir);
         if(hit && hit.collider && hit.collider.GetComponent<Rigidbody2D>() != null) {
             hit.collider.GetComponent<Rigidbody2D>().gravityScale = 1;
         }
 
         if( Input.GetMouseButtonUp (0)){
             grabbedObject = null;
         }
     }
 
     void FixedUpdate() {
         if(grabbedObject != null) {
            Vector3 mouseWorldPos3D = Camera.main.ScreenToWorldPoint(Input.mousePosition);
            mousePos2D = new Vector2(mouseWorldPos3D.x, mouseWorldPos3D.y);
 
            Vector2 dir = mousePos2D - grabbedObject.position;
 
             dir *= dragSpeed;
 
            grabbedObject.velocity = dir;
         }
     }
 }