I'm writing a very simple platformer, and i'm trying to get the script that controls the player right. I have the following so far:

using UnityEngine;
using System.Collections;

public class PlatformerControlScript: MonoBehaviour {
    public float GravityStrength = 5;
    public float JumpStrength = 1000;
    public float MovementSpeed = 500;
    public float XTolerance = 1;

    void FixedUpdate () 
    {
        float forceX = Input.GetAxis("Horizontal") * MovementSpeed;
        rigidbody.AddForce(new Vector3(forceX, 0, 0));
    }
    void OnCollisionStay(Collision collisionInfo)
    {
        if (collisionInfo.gameObject.tag == "Lava")
        {
            Die();
        }
        if (rigidbody.velocity.x <= XTolerance && rigidbody.velocity.x >= -XTolerance) //we are still enough to do.... something
        {
            if (Input.GetKeyDown(KeyCode.Space))
            {
                rigidbody.AddForce(new Vector3(0, JumpStrength, 0));
            }
        }
    }
    void Die()
    {
        //TODO: add code here
    }
}

There's some piping code for when i add in a lava tile to my platformer, but ignore that for now. Right now, i'm just trying to get my platformer guy to jump: If he is still on a surface, and i press space, he won't jump. The force i'm applying should be enough, yet it doesn't seem to be jumping. What's the recommended way to get something to "jump"?