How to increase the speed of an object's movement when it enters a trigger?

This is my first Unity3D assignment, and could use some help.

I’m basically making my own Rube Goldberg model, and I want to make the moving sphere speed up when it reaches this roller-coaster loop, as shown in my poorly drawn sketch. For example, when it collides with the green area, it is triggered to speed up so it would have enough momentum to reach the other end.

I can’t for the life of me find any tutorials anywhere, and I’m an absolute beginner at programming.

//Speed and acceleration variable
float speed, accel;

void OnTriggerEnter (Collider other)
    {
    //check if the other collider is tagged as "Area"
    if(other.tag == "Area")
        {
        speed += accel;
        }
    }

 //to move the object you can either use addforce or translate.

void FixedUpdate
    {
    GetComponent<Rigidbody>().AddForce(0, 0, speed * Time.DeltaTime);
    }

//or

void Update
    {
    transform.Translate(0, 0, speed * Time.DeltaTime);
    }