behavior error ,behavior error

I’m using c sharp the error reads as No MonoBehavior scripts in the file, or their names do no match the file name.

using UnityEngine;

public class PlayerMovement : MonoBehaviour {

public Rigidbody rb; 

// Update is called once per frame

void FixedUpdate () 
{
	rb.AddForce (0, 0, 2000 * Time.deltaTime); 
}

},
after trying to apply my script in unity i receive this error

The associated script can not be be loaded. Please fix any compile errors and assign a valid script.

also another error

No Mono Behavior scripts in the file, or their names do not match the file name.

I’m writing in c sharp on Mono Develop

using UnityEngine;

public class PlayerMovement : MonoBehaviour {

        public Rigidbody rb; 



void FixedUpdate () 
{
	rb.AddForce (0, 0, 2000 * Time.deltaTime); 
}

}

MonoBehaviours must be in a file of the same name - so the file holding this PlayerMovement class must be named PlayerMovement.cs