How to make movement relative to player rotation using Rigidbody.AddForce() in 3D Unity?

I'm making a 3D game and I was trying to make movement relative to player rotation for several hours, but I failed. Here's the code, maybe I made a mistake there. Also, player doesn't rotate.

public class PlayerScript : MonoBehaviour { private Quaternion rotat; //rotation of the camera public Transform cam; //camera bool IsOnGround = true; public float speed = 5; private Rigidbody rb; void Start() { rb = GetComponent<Rigidbody>(); } void FixedUpdate() { rotat = Quaternion.Euler(0, cam.rotation.y, 0); IsGrounded(); if (Input.GetKey(KeyCode.W)) { rb.AddForce(rotat * Vector3.forward * speed, ForceMode.Impulse); } if (Input.GetKey(KeyCode.A)) { rb.AddForce(rotat * Vector3.left * speed, ForceMode.Impulse); } if (Input.GetKey(KeyCode.S)) { rb.AddForce(rotat * Vector3.back * speed, ForceMode.Impulse); } if (Input.GetKey(KeyCode.D)) { rb.AddForce(rotat * Vector3.right * speed, ForceMode.Impulse); } if (Input.GetKey(KeyCode.Space) && IsOnGround == true) { IsOnGround = false; rb.AddForce(Vector3.up * (speed / 10), ForceMode.Impulse); } } public bool IsGrounded() { RaycastHit hit; float rayLength = 1.12f; if (Physics.Raycast(transform.position, Vector3.down, out hit, rayLength)) { IsOnGround = true; return true; } IsOnGround = false; return false; } }