rigidbody2D.AddForce() not working

using UnityEngine;
using System.Collections;

public class RocketScript : MonoBehaviour {
public Vector2 speed = new Vector2(0, 20);

public Vector2 downspeed = new Vector2(0, -20);
// Use this for initialization
void Start () {
	
}

// Update is called once per frame
void Update () {
	
	
	
	
	
}
void FixedUpdate () {
	if (Input.GetKeyDown (KeyCode.DownArrow))
					rigidbody2D.AddForce (downspeed);

	if (Input.GetKeyDown (KeyCode.UpArrow))
	    
					rigidbody2D.AddForce (speed);
	}	    

}
is my script but when i playtest it doesnt work-the sprite doesnt move (the rocket). Im not all that clear on how to use the addforce function and ive even read the docs but still not grasped it, so if anyone could help it would be very much appreciated.

I see you are using the “Input.GetKeyDown” instead of the “Input.GetKey”. When you use the KeyDown this only happens once when you first press the key, with GetKey it happens as long as you hold the key down. Another problem is that only 20 and -20 force is applied. That is not very much. I think you should scale thoose numbers up a bit to maybe 40 and -40. And if you really meant to write “Input.GetKeyDown” maybe 200 and -200.