Physics and Collisions

# CHAPTER 7

Physics and Collisions #

1. Introduction #

2. Learning Objectives #

• Explain the relationship between a Collider and a Rigidbody.

• Differentiate between a physical Collision and an overlapping Trigger.

• Use OnCollisionEnter to detect solid impacts.

• Use OnTriggerEnter to detect invisible zones (like coin pickups).

• Optimize collision logic using Unity Tags.

3. The Collider Component #

• A 3D model (the Mesh) is just for the player's eyes. The physics engine only cares about the Collider.

• Box Collider: Perfect for walls, crates, and floors. Extremely fast to calculate.

• Sphere Collider: Perfect for balls and projectiles.

• Capsule Collider: A cylinder with rounded ends. The industry standard for humanoid players so they slide up stairs smoothly.

4. The Rigidbody (Making it Dynamic) #

• A Collider makes an object solid.

• A Rigidbody hands the object over to the physics simulation.

5. Solid Collisions (OnCollisionEnter) #

// This script is attached to a bullet
void OnCollisionEnter(Collision hitData)
{
    // hitData contains info about what we just hit!
    if (hitData.gameObject.tag == "Enemy")
    {
        Debug.Log("Hit the enemy! Deal damage!");
        Destroy(gameObject); // Destroy the bullet
    }
}

6. Triggers (OnTriggerEnter) #

1. 1. Click the Collider component on the Coin.

1. 2. Check the box that says Is Trigger. The object instantly becomes a ghost!

1. 3. To detect the overlap in code, you use a different method:

// This script is attached to the player
void OnTriggerEnter(Collider other)
{
    if (other.tag == "Coin")
    {
        Debug.Log("Collected a coin!");
        Destroy(other.gameObject); // Destroy the coin
    }
}

7. Unity Tags (Identification) #

• If you don't use Tags, your player script will think it collected a coin every time it accidentally touches the floor or a wall!

• Always tag your important objects: "Player", "Enemy", "Coin", "Wall".

8. Visual Learning: The Physics Matrix #

How do two objects interact?
[Object A]               [Object B]               [Result]
Collider                 Collider                 Nothing (Both are static walls)
Collider + Rigidbody     Collider                 Solid crash! (A bounces off B)
Collider (Is Trigger)    Collider + Rigidbody     Ghost overlap! (OnTriggerEnter fires)

9. Best Practices #

• Never move Static Colliders: If a wall has a Collider but no Rigidbody, Unity assumes it will NEVER move, and optimizes it heavily. If you write a script to move that wall, Unity will panic and recalculate the entire level's physics, causing massive lag. If a wall needs to move (like a sliding door), you *must* give it a Rigidbody and check the "Is Kinematic" box!

10. Common Mistakes #

• Forgetting the Rigidbody on Triggers: For OnTriggerEnter to fire, at least one of the two objects involved MUST have a Rigidbody attached. If your player is walking through coins and the code isn't firing, ensure your Player has a Rigidbody!

11. Mini Project: The Coin Collector #

1. 1. Create a Cube (The Floor) and a Capsule (The Player). Give the Player a Rigidbody and a Movement script.

1. 2. Tag the Player as "Player" in the top left of the Inspector.

1. 3. Create a small Sphere (The Coin). Check the Is Trigger box on its Sphere Collider.

1. 4. Go to the top of the Inspector, click the Tag dropdown -> Add Tag. Add a tag named "Coin". Apply it to the Sphere.

1. 5. Create a script named CoinPickup and attach it to the Coin.

using UnityEngine;

public class CoinPickup : MonoBehaviour
{
    void OnTriggerEnter(Collider other)
    {
        // Did the Player touch this trigger?
        if (other.CompareTag("Player"))
        {
            Debug.Log("Coin Collected!");
            Destroy(gameObject); // Delete the coin
        }
    }
}

1. 6. Press Play and walk the player into the coin. It will vanish!

12. Practice Exercises #

1. 1. What checkbox must you tick on a Collider component to make it an invisible zone that players can walk through?

1. 2. Why is the Unity Tag system critical when writing collision code?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the mechanical difference between a solid Collision and a Trigger in Unity. Provide a real-world gameplay example for each scenario.

• Q: Explain the rule of moving Static Colliders. Why does moving a standard Collider without a Rigidbody cause immense CPU performance drops?

• Q: A developer writes if (hit.gameObject.name == "Enemy_01") inside a collision check. Why is relying on the GameObject's name a terrible practice, and what system should they use instead?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #