Need help increasing GameObject speed without affecting direction OnCollision.

Hello,

I have been trying to add gameplay features to my recently created Pong clone. I wanted my ball to increase in speed every time it bounced (entered collision with another object). I haven't been able to figure this out. I can only get to the RigidBody's velocity, but that is not what I want, since tinkering with this also has some sort of effect on my ball's direction. Here is my code;

 using UnityEngine;
 using System.Collections;
 
 public class BallScript : MonoBehaviour {
 
     public float ballVelocity = 3000;
     public float thrust = 500;
 
     Rigidbody rb;
     bool isPlay;
     int randInt;
 
     void Awake () {
         rb = gameObject.GetComponent<Rigidbody>();
         randInt = Random.Range(1,3);
     }
     
     void FixedUpdate () {
         if (Input.GetMouseButton(0)==true && isPlay == false) {
             transform.parent = null;
             isPlay = true;
             rb.isKinematic = false;
             if(randInt==1){
                 rb.AddForce (new Vector3 (ballVelocity, ballVelocity, 0));
             }
             if(randInt==2){
                 rb.AddForce (new Vector3 (ballVelocity, -ballVelocity, 0));
             }
 
     }
 }
     void OnCollisionEnter(Collision collision) {
         rb.AddForce (transform.forward * thrust);
     }
 }

As you can see, I have tried increasing the ball's speed by using "rb.AddForce (transform.forward * thrust);", however, this actually does nothing and the ball keeps moving at the same speed even after hitting other colliders. What am I missing here? Please help, this is just my second week using the program.