Why Error in player movement - Unity 3D C#

Question

I have a ball in my scene that I want to move it forward in a constantly speed, so I made this code for the ball:

public float speed = 100f;

    void FixedUpdate () {

        //rb.AddRelativeForce(Vector3.forward * speed * Time.deltaTime);
        transform.position += Vector3.forward * Time.deltaTime * speed;
        //rb.AddForce(0, 0, speed *Time.deltaTime);
        //rb.velocity = transform.forward * speed;
    }

NB: I've tried all the codes you see in the comments (//...)

The problem is sometimes the ball is destroying by itself, and sometimes the speed decrease, or when the ball collide with other object it go forward and backward in a weird movement!

Here's the entire code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
[RequireComponent(typeof(AudioSource))]
public class PlayerMovement : MonoBehaviour {

    public AudioSource audioSource1;
    public AudioSource audioSource2;
    public Transform BallDestroyEffect;
    public ParticleSystem BallCollectEffect;
    public ParticleSystem SpeedEffect;
    public ParticleSystem FireWorksEffect;
    public GameObject FloatingTextPrefab;
    public GameObject ContinueButton;

    public Rigidbody rb;
    public float speed = 100f;
    private int count;
    public Text countText;
    public Text winText;
    bool gameStart;

    private void Start()
    {
        BallCollectEffect.Stop();
        FireWorksEffect.Stop();
        ContinueButton.SetActive(false);
        rb = GetComponent<Rigidbody>();

        count = 0;
        setcountText();
        winText.text = "";
        audioSource1 = GetComponent<AudioSource>();
        audioSource2 = GetComponent<AudioSource>();
    }

    void FixedUpdate () {

        //rb.AddRelativeForce(Vector3.forward * speed * Time.deltaTime);
        //transform.position += Vector3.forward * Time.deltaTime * speed;
        //rb.AddForce(0, 0, speed *Time.deltaTime);
        //rb.velocity = transform.forward * speed;
        rb.MovePosition(transform.position + transform.forward * speed);
    }

    void OnCollisionEnter(Collision collision)
    {
        if (collision.gameObject.CompareTag ("Enemy"))
        {
            audioSource2.Play();
            SpeedEffect.Stop();
            rb.gameObject.SetActive(false);
            Instantiate(BallDestroyEffect, transform.position, transform.rotation);
            FindObjectOfType<GameManager>().EndGame();

        }
        if (collision.gameObject.CompareTag("End"))
        {
            ContinueButton.SetActive(true);
            winText.text = "Level Completed!";
            SpeedEffect.Stop();
            FireWorksEffect.Play();
        }
        if(collision.gameObject.CompareTag("Glass"))
        {
            audioSource1.Play();
            count = count + 3;
            setcountText();
            BallCollectEffect.Play();
            if (FloatingTextPrefab)
            {
                ShowFloatingText();
            }
        }
    }

    void ShowFloatingText()
    {
        Instantiate(FloatingTextPrefab, transform.position, Quaternion.identity, transform);
    }


    void setcountText ()  {
        countText.text = count.ToString();
    }
}

Do you have any suggestions about that?

Answer 1

Rigidbody.MovePosition()

This should be used if you want to continuously move a rigidbody in each FixedUpdate.

Set Rigidbody.position instead, if you want to teleport a rigidbody from one position to another, with no intermediate positions being rendered.

Note the difference in those two sentences.