Gravity is not working

using System.Collections; using UnityEngine.Scene$$anonymous$$anagement;

public class PlayerControlls : $$anonymous$$onoBehaviour { private float speed = 10.0f; private Rigidbody rb; // private float gravity = 9.8f; private int deaths; private int gemCount; private int nextScene; // CharacterController cont;

 // Use this for initialization
 void Start () {
     rb = GetComponent<Rigidbody>();
     //cont = gameObject.GetComponent<CharacterController>();

 }
 void OnLevelWasLoaded(int level) {
     nextScene = level + 1;
     }


 void OnTriggerEnter (Collider obj) {
     // kill player when they fall off
     if (obj.CompareTag("killZone")) {
         Scene$$anonymous$$anager.LoadScene (Scene$$anonymous$$anager.GetActiveScene ().name);
         deaths++;
     }

     // portal leads to next scene
     if (obj.CompareTag("portal")) {
         Scene$$anonymous$$anager.LoadScene (nextScene);
         deaths++;
     }

     //collects yellow gems
     if (obj.CompareTag("yellowGem")) {
         Destroy (obj.gameObject);
         gemCount++;
     }

 }

 public int getGemCount() {
     return gemCount;
 }


 void FixedUpdate () {
     Debug.Log (gemCount);
     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");


     Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical) ;


     rb.AddForce(movement * speed);

// if (!cont.isGrounded) { // movement.y -= gravity;

 //    }
 }

 void Update () {
     





 }

}