Space shooter tutorial, space ship keep moving and has interia

I just follow the tut and make my space ship move. However, it seems like the ship has a strong inertia and keeps moving even I release the move key. I hope my ship could stop as long as I release the move key. I am wondering why is it happening since I just follow the tut and get a different result. Here is my code:

public class GameController : MonoBehaviour {

 private Rigidbody rb;
 public float speed;

 void Start() {
     rb = GetComponent<Rigidbody>();
 }

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

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

}