null reference Exception object reference not set i did not understand the prob. please help me

public class PlayerMovment : MonoBehaviour {

 private Rigidbody rb;

 void start()
 {
     //accesing comppnent
     rb = GetComponent<Rigidbody>();
 }
 
 void FixedUpdate()
 {
     ///getting input from the player
     float moveHorizontal = Input.GetAxis("Horizontal");
     float moveVertical = Input.GetAxis("Vertical");

     //calculating the values
     Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
     
     //adding the input to the player
     rb.AddForce(movement);
 }

}