(Unity)カメラの向きとボールの進行方向を合致させる

(スクリプト)

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

public class Ball : MonoBehaviour
{
    private Rigidbody rb;
    public float speed;
    public GameObject cam;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {
        float moveH = Input.GetAxis("Horizontal");
        float moveV = Input.GetAxis("Vertical");
        Vector3 movement = new Vector3(moveH, 0, moveV);

        // カメラの向きとボールの進行方向を合致させる。
        Vector3 desiredMove = cam.transform.forward * movement.z + cam.transform.right * movement.x;

        rb.AddForce(desiredMove * speed);
    }
}

(設定)