NullReferenceException

using UnityEngine;
using System.Collections;
public class Player_movement : MonoBehaviour {

public float Speed = 10;
public GameObject cube;
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);
}
}




There was no error and everything was good but when I pressed the play button, The console started filling up with error, once i clicked on the error it showed me that rb.AddForce (movement * Speed) is the problem but I can't see any problem there, can someone help me?

 

Does the gameObject have a rigidbody on it. Also use

Code (csharp):

1.  tags when posting code

 

http://docs.unity3d.com/Manual/NullReferenceException.html