Jumping Doesn't Work

Hi, I am pretty new to using Unity and I decided to create a basic program to practice some of my learned skills with Unity. I made a platform and a ball that moves around with the arrow keys and I tried to make a jumping feature as well, but the jumping feature doesn't seem to work. I have created a Floor tag and applied it to the platform, and I added the platform as the game object into my sphere script. I do not get any errors, however. What am I doing wrong?

 using UnityEngine;
 using System.Collections;
 
 public class Controller : MonoBehaviour {
 
     public int speed;
     public GameObject gameObject;
 
     private Rigidbody rb;
     private bool jumpAvailable;
 
     Vector3 up = new Vector3 (0, 0, 1);
     Vector3 down = new Vector3 (0, 0, -1);
     Vector3 left = new Vector3 (-1, 0, 0);
     Vector3 right = new Vector3 (1, 0, 0);
     Vector3 jump = new Vector3 (0, 3, 0);
 
     void Start () {
         rb = GetComponent<Rigidbody> ();
     }
 
     void FixedUpdate () {
 
         if (Input.GetKey ("up")) {
             rb.AddForce (up * speed * Time.deltaTime);
         }
         if (Input.GetKey ("down")) {
             rb.AddForce (down * speed * Time.deltaTime);
         }
         if (Input.GetKey ("left")) {
             rb.AddForce (left * speed * Time.deltaTime);
         }
         if (Input.GetKey ("right")) {
             rb.AddForce (right * speed * Time.deltaTime);
         }
         if (Input.GetKey ("space")) {
             if (jumpAvailable == true) {
                 rb.AddForce (jump * speed * Time.deltaTime);
                 jumpAvailable = false;
             }
 
         }
 
     }
 
     void OnTriggerEnter (Collider other) {
 
         if (other.gameObject.CompareTag ("Floor")) {
             jumpAvailable = true;
         }
 
     }
 
 }