Keep the Speed

Lesson 3 of 19 in Coddy's Unity Game: Brick Breaker course.

A Brick Breaker ball never slows down. Its PhysicsMaterial does most of the work: bounciness 1 keeps all of a bounce's speed, and friction 0 stops the walls from dragging it. The walls have no material, and Unity averages the two colliders' values by default: the ball's 1 and a wall's 0 make a bounce at half speed. bounceCombine = Maximum and frictionCombine = Minimum let the ball's own values win.

The physics is still not exact: a moving paddle adds its speed, and small errors add up over hundreds of bounces. So every physics step the ball keeps its direction and gets its speed set back:

rb.linearVelocity = rb.linearVelocity.normalized * 7f;

A ball slowed to 2 units a second flies at 7 again on the next step.

Challenge

Easy

Make the ball bounce forever at one speed.

In Ball's Awake(), make new PhysicsMaterial("Bouncy") with bounciness 1, both frictions 0, bounceCombine = PhysicsMaterialCombine.Maximum and frictionCombine = PhysicsMaterialCombine.Minimum; set it on GetComponent<Collider>().material.
Add FixedUpdate(): if launched, rb.linearVelocity = rb.linearVelocity.normalized * speed;

Press Play and launch: the ball bounces off every wall without slowing. Then Check.

Try it yourself

using UnityEngine;
using UnityEngine.InputSystem;

public class Ball : MonoBehaviour
{
    public Transform paddle;
    public float speed = 9f;

    Rigidbody rb;
    bool launched;

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

    void Update()
    {
        if (launched) return;
        transform.position = paddle.position + Vector3.up * 0.5f;
        if (Keyboard.current.spaceKey.wasPressedThisFrame) Launch(new Vector3(0.3f, 1f, 0f));
    }

    public void Launch(Vector3 direction)
    {
        launched = true;
        rb.linearVelocity = direction.normalized * speed;
    }
}