Why is this AddForce Error coming up?

I am trying to complete my enemy animation for the opposite AI to move towards my player but Monodevel keeps popping up with this error message.

using UnityEngine; using System.Collections;

public class AI1 : MonoBehaviour {

 private Vector3 Player;
 private Vector2 Playerdirection;
 private float Xdif;
 private float Ydif;
 private float speed;

 void Start (){
     speed = 10;
 }

 void Update () {
     Player = GameObject.Find ("Player").transform.position;

     Xdif = Player.x - transform.position.x;
     Ydif = Player.y - transform.position.y;

     Playerdirection = new Vector2 (Xdif, Ydif);

     GetComponent<Rigidbody2D>().Addforce (Playerdirection.normalized * speed);

ERROR: Assets/Scripts/AI1.cs(24,45): error CS1061: Type UnityEngine.Rigidbody2D' does not contain a definition for Addforce' and no extension method Addforce' of type UnityEngine.Rigidbody2D' could be found (are you missing a using directive or an assembly reference?)

Any suggestions?

Thanks, Confused