my unity gravity isn't functioning properly, how do I fix it?

Question

I'm new to this, I'm making an obstacle dodging game, and part of the losing conditions is that if the player object falls below -1 on the y axis then the player loses the game, but every time I run the game and intentionally go off the edge of the ground, the player object just floats. I've double checked that the Rigidbody for the player uses gravity, and even checked my scripts for logic errors, nothing has worked. got any ideas on how to fix it?

I've already added the code rb.useGravity = true;

here's the script for the player's movement: `

using UnityEngine;

public class movement : MonoBehaviour
{
    public Rigidbody rb;
    public float ForwardMomentum = 1000f
    public float SidewaysMomentum = 500f

void FixedUpdate()
{
    rb.useGravity = true;
    rb.AddForce(0, 0, FowardMomentum * Time.deltaTime);

    if (Input.GetKey("d"))
    {
        rb.AddForce(SidewaysMomentum * Time.deltaTime, 0, 0, ForceMode.VelocityChange);
    }

   else if (Input.GetKey("a"))
    {
        rb.AddForce(-SidewaysMomentum * Time.deltaTime, 0, 0, ForceMode.VelocityChange);
    }
    
}

To make a reproducible example for this, all you need to do is:

1. Add a cube game object (I believe that the box collider and mesh renderer should already be loaded in there)
2. Add the Rigidbody component and tick use gravity (As well as ticking freeze rotation on the x axis)
3. Add the new script with the code from above (please fix any errors you find to make it work and let me know how you did it :D)
4. Add a ground floor and just make it so the player cube is sitting on top of it
5. then just run it a couple of times and see how it goes, if nothing is out of the ordinary then idk what's going on with my unity