game objects leaving trail of it's self

so I’m doing the space shooter tutorial make the changes need to get the player to move.

using UnityEngine;
using System.Collections;

[System.Serializable]
public class Boundary
{
public float xMin, xMax, zMin, zMax;
}

public class PlayerController : MonoBehaviour
{
public float speed;
public float tilt;
public Boundary boundary;
private Rigidbody rb;
void Start()
{
rb = GetComponent();
}

void FixedUpdate()
{
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");

    Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
    //rb.velocity = movement * speed;
    rb.AddForce(movement * speed);

    rb.position = new Vector3
    (
        Mathf.Clamp(rb.position.x, boundary.xMin, boundary.xMax),
        0.0f,
        Mathf.Clamp(rb.position.z, boundary.zMin, boundary.zMax)
    );

    rb.rotation = Quaternion.Euler(0.0f, 0.0f, rb.velocity.x * -tilt);
}

}
I change the //rb.velocity = movement * speed; for rb.AddForce(movement * speed); because velocity only moved it like a fraction of a inch and not move any further expect to origin. so I’m alittle confused on what to try.

It looks like your camera isn’t clearing properly. Make sure that clear flags is set to Skybox or Solid Color75647-img.jpg