An object reference is required

I started using unity and making Roll-a-ball i finish the script but i get this message

“Assets/Script/PlayerController.cs(18,19): error CS0120: An object reference is required to access non-static member `UnityEngine.Rigidbody.AddForce(UnityEngine.Vector3, UnityEngine.ForceMode)'”

Anyone,can help me ?

This is my script

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

//Use this for initilization
void Start () {

}

//Update is called once per frame
void FixedUpdate (){
		float moveHorizontal = Input.GetAxis ("Horizontal");
		float moveVertical = Input.GetAxis("Vertical");

		Vector2 movement = new Vector2 (moveHorizontal, 0.0f, moveVertical);
		Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);

	Rigidbody.AddForce(movement);
	}
}

Rigidbody is the name of the class, and rigidbody is the variable referencing the rigidbody object of a gameObject. Try lowercase rigidbody, and it should work.