Impulse Force not smooth

I'm trying to make falling rocks with different speeds and directions

here is the code , it is working perfectly however The rocks aren't falling smoothly. am I missing something

gravity scale = 0

 void Start () {
         int MaxForce = 0;
         int MinForce = 0;
         switch (GameManager.Instance.mode)
         {
             case GameMode.Easy:
                 MaxForce = 10;
                 MinForce = 5;
                 break;
             case GameMode.Medium:
                 MaxForce = 15;
                 MinForce = 7;
                 break;
             case GameMode.Hard:
                 MaxForce = 20;
                 MinForce = 9;
                 break;
             default:
                 break;
         }
         Vector2 Force = new Vector2(Random.Range(MinForce-3, MaxForce-3) * Random.Range(-1f, 1f), Random.Range(-MaxForce, -MinForce));
 
         GetComponent<Rigidbody2D>().AddForce(Force,ForceMode2D.Impulse);
     
     }