Move ball inside pipe like tunel

i want to ball move inside pipe like hollow cylinder.Ball constantly moving forward and by arrows we can move ball 180 degree left or in curved path right like 102753-question.png picture.i write that can move ball in curved shape but i can’t able to move forward me

           currentRotation += Input.GetAxis("Horizontal")*Time.deltaTime*200;
	currentRotation=Mathf.Clamp (currentRotation, -180, 0);
	rotation.eulerAngles = new Vector3(0,0, currentRotation);
	print (rotation * radius);

	transform.position = rotation*radius ;

hi, can be done with Rigidbody.AddForce like this :

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class moveball : MonoBehaviour {
    public float speed=2.0f;
    void FixedUpdate()
        float moveHorizontal = Input.GetAxis( "Horizontal" );
        float moveVertical = Input.GetAxis( "Vertical" );
        Vector3 movement = new Vector3( moveHorizontal , 0.0f , moveVertical );
        GetComponent<Rigidbody>().AddForce( movement * speed * Time.deltaTime );