Physics Systems in Unity

# CHAPTER 16

Physics Systems in Unity #

1. Introduction #

2. Learning Objectives #

• Configure a Unity Rigidbody for dynamic simulation.

• Differentiate between Update() and FixedUpdate() in Unity C#.

• Utilize Unity's Physics.Raycast for instant hit detection.

• Apply Unity PhysicMaterial to control friction and bounciness.

• Use Unity's CharacterController for Kinematic player movement.

3. The Unity Rigidbody #

• Mass: Unity assumes 1 Unit = 1 Meter, and Mass = Kilograms.

• Drag & Angular Drag: Air resistance.

• Use Gravity: A simple checkbox. Unchecking it allows objects to float in space while still bouncing off walls.

• Is Kinematic: Checking this tells PhysX: "I will move this object using my C# script, but I still want it to push other rigidbodies around." (Used for moving platforms).

• Interpolate: If your camera follows a Rigidbody and it looks jittery, turning on Interpolation forces Unity to smooth out the visual rendering between physics frames.

4. The FixedUpdate Loop #

• Update() runs every time a frame is drawn (maybe 60 times a second, maybe 144).

• FixedUpdate() runs on a strict, unyielding timer (by default, exactly 50 times a second, or every 0.02 seconds).

public Rigidbody rb;

// BAD: Will result in inconsistent, jittery jumps
void Update() {
    if (Input.GetKeyDown(KeyCode.Space)) rb.AddForce(Vector3.up * 10f);
}

// GOOD: Frame-rate independent physics calculation
void FixedUpdate() {
    if (jumpRequested) {
        rb.AddForce(Vector3.up * 10f, ForceMode.Impulse);
        jumpRequested = false;
    }
}

5. Unity Colliders and Materials #

1. 1. Right-click in the Project window -> Create -> Physic Material.

1. 2. Set Bounciness (0 to 1).

1. 3. Set Dynamic Friction (sliding friction).

1. 4. Drag this material into the Material slot on your Collider component.

6. The CharacterController Component #

• It automatically handles sliding against walls.

• It automatically steps up stairs (using the "Step Offset" setting).

• It handles slopes (using the "Slope Limit" setting).

7. Visual Learning: Unity Physics Debugging #

1. 1. At the top of the Scene View, click the Gizmos button. This draws green outlines around all invisible Colliders.

1. 2. In C#, use Debug.DrawRay(startPos, direction, Color.red) to visually draw your mathematical Raycasts in the Scene View!

8. Best Practices #

• Collision Matrices in Unity: Go to Edit -> Project Settings -> Physics. At the bottom, you will see a massive grid of checkboxes. Uncheck interactions (e.g., "PlayerProjectiles" vs "PlayerProjectiles") to instantly optimize your Unity game.

9. Common Mistakes #

• Scaling Colliders via the Transform: If you have a SphereCollider, and you change the GameObject's Transform.Scale from (1, 1, 1) to (1, 5, 1), you have created an oval. PhysX hates non-uniform scales for spheres and will often glitch or throw errors. Always scale the SphereCollider's *internal* Radius property, not the GameObject's Transform!

10. Mini Project: Unity Physics Playground #

using UnityEngine;

[RequireComponent(typeof(Rigidbody))]
public class UnityPhysicsPlayer : MonoBehaviour
{
    private Rigidbody rb;
    public float jumpForce = 5f;
    private bool shouldJump = false;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
        
        // Optimize: Prevent the physics engine from tipping the player over!
        rb.constraints = RigidbodyConstraints.FreezeRotationX | RigidbodyConstraints.FreezeRotationZ;
    }

    void Update()
    {
        // 1. Get Input in Update (because inputs are lost if checked in FixedUpdate!)
        if (Input.GetKeyDown(KeyCode.Space))
        {
            shouldJump = true;
        }
    }

    void FixedUpdate()
    {
        // 2. Apply Physics in FixedUpdate
        if (shouldJump)
        {
            // Shoot a Unity Raycast down 1.1 meters to check for the floor
            if (Physics.Raycast(transform.position, Vector3.down, 1.1f))
            {
                rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
            }
            shouldJump = false; // Reset the trigger
        }
    }
}

11. Practice Exercises #

1. 1. What is the name of the C++ physics engine that runs under the hood of Unity 3D?

1. 2. Why is it a critical error to place Rigidbody.AddForce() calls inside the standard Update() method?

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the mechanical difference between Unity's Update() and FixedUpdate() loops. Provide a scenario where mixing them up results in a gameplay bug.

• Q: How do you configure a Unity PhysicMaterial to create a perfect, endlessly bouncing rubber ball? What properties are changed, and what are their values?

• Q: A junior developer uses Transform.Translate to move an enemy through a wall, bypassing the Rigidbody entirely. Explain why this breaks the PhysX pipeline and what method they should use instead.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #