How to change addForce direction depending on camera direction

Hey so I was making a game where the player is almost the same where it is in the 'roll a ball tutorial' as it is a ball the rolls around when the player does WASD or arrow keys, I also added a script to the camera to make it orbit around the player when you move the mouse. But the trouble is that if I move my mouse & press W, instead of the ball rolling away from the camera, it rolls in the same direction as before. If possible I would prefer to keep it as the ball 'rolls' around rather than just translating is it has that 'inertia' & behaves just like a real ball would. Here is the code for the player movement:

 public static float speed;
 private Rigidbody rb;

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

 void FixedUpdate()
 {
     float moveHorizontal = Input.GetAxis("Horizontal");
     float moveVertical = Input.GetAxis("Vertical");
     Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
     rb.AddForce(movement * speed);
 } 

& here is the code for the camera:

 public GameObject player;
 public Transform plyrTransform;
 public Transform camTransform;
 public Camera cam;
 private float distance = 10.0f;
 private float currentX = 0.0f;
 private float currentY = 0.0f;
 public float sensitivity = 1.0f;
 private float sensitivityX = 0.0f;
 private float sensitivityY = 0.0f;

 void Start () {
     camTransform = transform;
     sensitivityX = sensitivity;
     sensitivityY = sensitivity * 4;
 }

 void Update () {
     currentX += Input.GetAxis ("Mouse X") * sensitivityX;
     currentY += Input.GetAxis ("Mouse Y") * sensitivityY;
     distance += Input.GetAxis ("Mouse ScrollWheel");
 }

 void LateUpdate () {
     Vector3 dir = new Vector3 (0, 0, -distance);
     Quaternion rotation = Quaternion.Euler (currentY, currentX, 0);
     camTransform.position = plyrTransform.position + rotation * dir;
     transform.LookAt (player.transform);
 }

P.S, sorry if some of the code is unnecessary & messy, I am new to C# & follow a lot of tutorials.