Physics Programming Basics

# CHAPTER 9

Physics Programming Basics #

1. Introduction #

2. Learning Objectives #

• Understand the role of a Rigidbody component.

• Apply physical forces via code (e.g., AddForce).

• Use the FixedUpdate loop for stable physics calculations.

• Detect solid collisions using OnCollisionEnter.

• Detect overlapping ghost collisions using OnTriggerEnter.

3. The Rigidbody (Giving Objects Mass) #

• Once an object has a Rigidbody, you should *never* move it using transform.Translate() (teleporting).

• Instead, you must push it using physics forces.

4. Applying Forces (AddForce) #

class PhysicsPlayer
{
    // Reference to the engine's physics component
    public Rigidbody rb; 
    public float jumpForce = 500f;

    void Update()
    {
        if (Input.GetKeyDown("space"))
        {
            // Push the object upwards (Y-axis) with an Impulse force
            rb.AddForce(new Vector3(0, jumpForce, 0), ForceMode.Impulse);
        }
    }
}

5. The FixedUpdate Loop (The Physics Tick) #

• Engines solve this with FixedUpdate.

• FixedUpdate() runs at a locked, steady tick rate (usually exactly 50 times a second), regardless of your monitor's framerate.

• *Golden Rule:* If you are constantly applying physics forces (like pushing a car forward), that code MUST go in FixedUpdate().

6. Collision Detection #

• Game engines provide built-in event methods.

• OnCollisionEnter: Fires the exact frame two solid objects smash into each other.

// This is automatically called by the engine upon impact
void OnCollisionEnter(Collision hitData)
{
    // Check the "Tag" of the object we just hit
    if (hitData.gameObject.tag == "Enemy")
    {
        Console.WriteLine("Ouch! Hit an enemy!");
        TakeDamage(10);
    }
}

7. Triggers (Ghost Collisions) #

• By checking the "Is Trigger" box on a collider, it becomes a ghost.

• To detect it in code, use OnTriggerEnter.

void OnTriggerEnter(Collider other)
{
    if (other.tag == "Coin")
    {
        gold += 1;
        Destroy(other.gameObject); // Delete the coin
    }
}

8. Visual Learning: Collision vs. Trigger #

[ COLLISION ] (Solid impact, physics engine bounces them away)
Player [======] -> SMAASH! <- [======] Brick Wall
Action: OnCollisionEnter() fires.

[ TRIGGER ] (Ghost overlap, no physical bounce)
Player [======] -> walks into -> (::::) Gas Cloud
Action: OnTriggerEnter() fires.

9. Best Practices #

• Never Use DeltaTime in FixedUpdate: Because FixedUpdate runs at a locked interval, the time between ticks is already perfectly constant. Multiplying by Time.deltaTime inside FixedUpdate is not only unnecessary, it will actually break your math.

10. Common Mistakes #

• Teleporting Rigidbodies: A beginner attaches a Rigidbody to a car, but writes transform.position.z += 5; in the Update loop. The car moves, but ignores gravity, flies through walls, and jitters uncontrollably. *Rule:* If an object has a Rigidbody, you must move it via rb.AddForce() or rb.velocity, NOT by changing its transform coordinates directly.

11. Mini Project: Build a Physics Bouncer #

class BouncyBall
{
    Rigidbody rb;

    void Start()
    {
        // Get the Rigidbody attached to this object
        rb = GetComponent<Rigidbody>(); 
    }

    void Update()
    {
        // Push the ball Right
        if (Input.GetKeyDown("d"))
        {
            rb.AddForce(new Vector3(300, 0, 0));
        }
    }

    // Detect when the ball hits the floor or a wall
    void OnCollisionEnter(Collision hit)
    {
        if (hit.gameObject.tag == "Wall")
        {
            Console.WriteLine("Boing! Hit the wall.");
            PlayBounceSound();
        }
    }
}

12. Practice Exercises #

1. 1. What is the fundamental difference between OnCollisionEnter and OnTriggerEnter?

1. 2. Why is the FixedUpdate loop used instead of the Update loop when applying continuous physics forces?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the mathematical dangers of mixing Transform movement (teleporting) with a Rigidbody component. How does the physics engine react?

• Q: A junior programmer writes rb.AddForce(Vector3.up * speed * Time.deltaTime); inside the FixedUpdate() method. Identify the mathematical flaw in this code.

• Q: Walk me through the execution order of Update() vs FixedUpdate(). How many times might FixedUpdate() be called during a single frame if the game drops to 10 FPS?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #