【Unity2D】オブジェクトをドラッグして動かし、ドロップと同時に重力をつけて落とす方法

using UnityEngine;
using System.Collections;

public class BakudanController : MonoBehaviour
{
    Vector3 screenPoint;
    Vector3 offset;

    Rigidbody2D rb;

    void Start()
{
        rb = this.GetComponent<Rigidbody2D>();
}

    void OnMouseDown()
    {
        this.screenPoint = Camera.main.WorldToScreenPoint(transform.position);
        this.offset = transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
        rb.gravityScale = 0.5f;

    }

    void OnMouseDrag()
    {
        Vector3 currentScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
        Vector3 currentPosition = Camera.main.ScreenToWorldPoint(currentScreenPoint) + this.offset;
        transform.position = currentPosition;
    }
}