Unity object not following mouse position

Question

I have created a object and it is attached to a spring joint 2D and then I have created a script on it so it follows the mouse position. But it is not happening so and in fact weird thing happens. When I click on object it appears at centre of the screen. Please have a look at my code. And why cant I use transform.position with it. Even more weird things happen. Object has a collider and rigidbody2d attached to it

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Pulling : MonoBehaviour {

    private Rigidbody2D rigidbody;
    private bool ispressed = false;

    void Start(){
        rigidbody = GetComponent<Rigidbody2D> ();
    }

    void Update(){

        if (ispressed) {
            rigidbody.position = Camera.main.ScreenToWorldPoint (Input.mousePosition);
        }
    }

    void OnMouseDown(){
        ispressed = true;
        rigidbody.isKinematic = true;
    }

    void OnMouseUp(){
        ispressed = false;
        rigidbody.isKinematic = false;
    }
}

Answer 1

The problem is that when you use Input.mousePosition as the screen point, its world point will be on the camera in z axis instead of on a further plane with a higher z.

You can solve this by assigning the z value of the screen point manually.

rigidbody.position = Camera.main.ScreenToWorldPoint (new Vector3(Input.mousePosition.x, Input.mousePosition.y, z));

where z is the distance of camera from the desired plane. (naturally z = -Camera.main.z)

Note that if the camera is aligned with z axis this will work, but if it's angled then ScreenToWorldPoint is not suitable and you need to find another workaround.