first time coding, getting "NullReferenceException: Object reference not set to an instance of an object" error

making the beginner rollaball game from the tutorials and I copied the code exactly as they show it and I keep getting this error NullReferenceException: Object reference not set to an instance of an object

heres the code

 using UnityEngine;
 using System.Collections;
 
 public class playercontroller : MonoBehaviour {
 
     public 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);
     }

just trying to get the ball to move, its saying the "rb.AddForce (movement);" is where im having an error

any help would be greatly appreciated!