Physics & Collision

Description

This skill covers Unity's physics engines (PhysX for 3D, Box2D for 2D), including rigidbodies, colliders, triggers, raycasting, physics materials, joints, and collision layers. Proper physics setup is essential for responsive gameplay — from basic collision detection to complex ragdolls and destruction systems.

When To Use

•Adding physics-based movement or interactions to objects
•Setting up collision detection between game objects
•Implementing raycasting for shooting, selection, or line-of-sight
•Creating trigger zones (checkpoints, damage areas, pickups)
•Building joints and constraints (doors, chains, bridges)
•Configuring physics layers to control what collides with what
•Optimizing physics performance

Prerequisites

•Unity 6 (6000.x)
•02 — C# Scripting fundamentals
•Understanding of Unity's component model

Instructions

1. Rigidbody Fundamentals

3D Rigidbody

csharp

// Rigidbody setup via code (usually configured in Inspector)
var rb = gameObject.AddComponent<Rigidbody>();
rb.mass = 1f;                                    // kilograms
rb.linearDamping = 0.5f;                         // air resistance (was "drag")
rb.angularDamping = 0.05f;                       // rotational resistance
rb.useGravity = true;
rb.interpolation = RigidbodyInterpolation.Interpolate; // smooth visual movement
rb.collisionDetectionMode = CollisionDetectionMode.ContinuousDynamic; // fast objects
rb.constraints = RigidbodyConstraints.FreezeRotationX | RigidbodyConstraints.FreezeRotationZ;

Body Types

Type	Use Case	Behavior
Dynamic	Players, projectiles, physics objects	Fully simulated — gravity, forces, collisions
Kinematic	Moving platforms, elevators, animated objects	Moved via code, unaffected by forces, still triggers collisions
Static (no Rigidbody)	Walls, floors, environment	Never moves — adding a collider without Rigidbody makes it static

Rule: Every moving object with a collider MUST have a Rigidbody. Moving a static collider forces PhysX to rebuild spatial data structures — extremely expensive.

Applying Forces

csharp

// All force application must happen in FixedUpdate()
private void FixedUpdate()
{
    // Continuous force (acceleration) — good for movement
    _rb.AddForce(Vector3.forward * _thrust, ForceMode.Force);
    
    // Instant impulse — good for jumps, explosions
    _rb.AddForce(Vector3.up * _jumpForce, ForceMode.Impulse);
    
    // Direct velocity change — good for knockback
    _rb.AddForce(_knockbackDir * _knockbackPower, ForceMode.VelocityChange);
    
    // Torque (rotation)
    _rb.AddTorque(Vector3.up * _turnSpeed, ForceMode.Force);
    
    // Move to position (kinematic-style, respects physics)
    _rb.MovePosition(transform.position + _velocity * Time.fixedDeltaTime);
    _rb.MoveRotation(Quaternion.Euler(0f, _targetAngle, 0f));
}

ForceMode	Description	Use Case
`Force`	Continuous, mass-dependent (N)	Movement, thrust
`Impulse`	Instant, mass-dependent (N·s)	Jump, explosion
`Acceleration`	Continuous, mass-independent (m/s²)	Consistent movement regardless of mass
`VelocityChange`	Instant, mass-independent (m/s)	Knockback, teleport-velocity

2. Colliders

Collider Types

Collider	Cost	Use Case
BoxCollider	Cheapest	Crates, walls, floors, rough shapes
SphereCollider	Cheapest	Pickups, trigger zones, rough characters
CapsuleCollider	Cheap	Characters, bullets, cylinders
MeshCollider (Convex)	Medium	Complex props (max 255 triangles)
MeshCollider (Non-Convex)	Expensive	Static environment only — cannot be dynamic

Performance rule: Prefer primitives (box, sphere, capsule). Use compound colliders (multiple primitives on child objects) instead of mesh colliders when possible.

Compound Colliders

code

Character GameObject (Rigidbody)
├── Body (CapsuleCollider)
├── Head (SphereCollider)
├── Shield (BoxCollider)
└── Weapon (BoxCollider, IsTrigger = true)

Place colliders on child GameObjects. They all register as part of the parent's Rigidbody.

Trigger Colliders

Set isTrigger = true to create pass-through detection zones:

csharp

public class DamageZone : MonoBehaviour
{
    [SerializeField] private float _damagePerSecond = 10f;
    [SerializeField] private DamageType _damageType;
    
    private void OnTriggerStay(Collider other)
    {
        if (other.TryGetComponent<IDamageable>(out var target))
        {
            target.TakeDamage(_damagePerSecond * Time.fixedDeltaTime, _damageType);
        }
    }
}

3. Collision Callbacks

Collision Matrix

	Static Collider	Rigidbody Collider	Kinematic Rigidbody	Trigger (Static)	Trigger (Rigidbody)	Trigger (Kinematic)
Static Collider	—	Collision	—	—	—	—
Rigidbody Collider	Collision	Collision	Collision	Trigger	Trigger	Trigger
Kinematic Rigidbody	—	Collision	—	Trigger	Trigger	Trigger

Collision Events

csharp

// For non-trigger colliders (physical contact)
private void OnCollisionEnter(Collision collision)
{
    // Access contact points
    ContactPoint contact = collision.GetContact(0);
    Vector3 hitPoint = contact.point;
    Vector3 hitNormal = contact.normal;
    float impactForce = collision.impulse.magnitude;
    
    // Check what we hit
    if (collision.gameObject.CompareTag("Enemy"))
    {
        // Handle enemy collision
    }
    
    // Spawn impact VFX at contact point
    Instantiate(_impactVfx, hitPoint, Quaternion.LookRotation(hitNormal));
}

private void OnCollisionStay(Collision collision) { /* Ongoing contact */ }
private void OnCollisionExit(Collision collision) { /* Contact ended */ }

// For trigger colliders (pass-through detection)
private void OnTriggerEnter(Collider other) { /* Object entered trigger */ }
private void OnTriggerStay(Collider other)  { /* Object inside trigger */ }
private void OnTriggerExit(Collider other)  { /* Object left trigger */ }

4. Raycasting

Raycasting is essential for shooting, line-of-sight, ground detection, and UI interaction.

Basic Raycasts

csharp

public class WeaponRaycast : MonoBehaviour
{
    [SerializeField] private float _range = 100f;
    [SerializeField] private float _damage = 25f;
    [SerializeField] private LayerMask _hitLayers;
    [SerializeField] private Transform _muzzle;
    
    public void Shoot()
    {
        if (Physics.Raycast(_muzzle.position, _muzzle.forward, out RaycastHit hit, 
            _range, _hitLayers, QueryTriggerInteraction.Ignore))
        {
            Debug.Log($"Hit: {hit.collider.name} at {hit.point}");
            Debug.DrawLine(_muzzle.position, hit.point, Color.red, 1f);
            
            // Apply damage
            if (hit.collider.TryGetComponent<IDamageable>(out var target))
            {
                target.TakeDamage(_damage);
            }
            
            // Spawn decal at hit point
            SpawnDecal(hit.point, hit.normal);
        }
    }
}

SphereCast (Thicker raycast — more forgiving aim)

csharp

// Great for aim assist or melee detection
if (Physics.SphereCast(origin, radius: 0.3f, direction, out RaycastHit hit, 
    maxDistance, layerMask))
{
    // Hit something within a 0.3m radius tube
}

RaycastAll and RaycastNonAlloc

csharp

// RaycastAll — allocates array (use sparingly)
RaycastHit[] hits = Physics.RaycastAll(origin, direction, maxDistance, layerMask);

// RaycastNonAlloc — zero allocation (preferred in Update loops)
private readonly RaycastHit[] _hitBuffer = new RaycastHit[10];

private void DetectObjects()
{
    int hitCount = Physics.RaycastNonAlloc(
        transform.position, transform.forward, _hitBuffer, 50f, _detectionMask);
    
    for (int i = 0; i < hitCount; i++)
    {
        ProcessHit(_hitBuffer[i]);
    }
}

Overlap Queries (Area detection)

csharp

// Sphere overlap — find all colliders in a sphere
Collider[] results = new Collider[20];
int count = Physics.OverlapSphereNonAlloc(
    transform.position, explosionRadius, results, affectedLayers);

for (int i = 0; i < count; i++)
{
    // Apply explosion force
    if (results[i].TryGetComponent<Rigidbody>(out var rb))
    {
        rb.AddExplosionForce(explosionForce, transform.position, explosionRadius, 
            upwardsModifier: 1f, ForceMode.Impulse);
    }
}

// Box overlap
Physics.OverlapBoxNonAlloc(center, halfExtents, results, orientation, layerMask);

// Capsule overlap
Physics.OverlapCapsuleNonAlloc(point0, point1, radius, results, layerMask);

5. Physics Materials

Physics Materials control surface friction and bounciness.

code

Create: Assets → Create → Physic Material

Common presets:
├── Ice.physicMaterial       — Dynamic Friction: 0.05, Static Friction: 0.05, Bounciness: 0
├── Rubber.physicMaterial    — Dynamic Friction: 0.8,  Static Friction: 0.9,  Bounciness: 0.8
├── Metal.physicMaterial     — Dynamic Friction: 0.3,  Static Friction: 0.4,  Bounciness: 0.2
├── Wood.physicMaterial      — Dynamic Friction: 0.5,  Static Friction: 0.6,  Bounciness: 0.1
└── Bouncy.physicMaterial    — Dynamic Friction: 0.2,  Static Friction: 0.2,  Bounciness: 1.0
                               Bounce Combine: Maximum

Property	Description
Dynamic Friction	Friction when sliding (0 = ice, 1 = sandpaper)
Static Friction	Friction when stationary (resists initial movement)
Bounciness	How much velocity is retained on bounce (0 = none, 1 = full)
Friction Combine	How two surfaces combine: Average, Minimum, Maximum, Multiply
Bounce Combine	How two surfaces combine bounce values

6. Joints

Joints constrain how rigidbodies relate to each other.

Joint	Use Case	Example
FixedJoint	Weld two objects together	Sticky projectiles, object attachment
HingeJoint	Rotation around one axis	Doors, flippers, pendulums
SpringJoint	Elastic connection	Bungee cords, suspension
ConfigurableJoint	Full control over all axes	Custom constraints, ragdolls
CharacterJoint	Humanoid ragdoll joints	Ragdoll limbs

Hinge Joint Example (Door)

csharp

public class PhysicsDoor : MonoBehaviour
{
    [SerializeField] private float _openAngle = 90f;
    [SerializeField] private float _springForce = 50f;
    [SerializeField] private float _damping = 5f;
    
    private HingeJoint _hinge;
    private bool _isOpen;
    
    private void Awake()
    {
        _hinge = GetComponent<HingeJoint>();
    }
    
    public void ToggleDoor()
    {
        _isOpen = !_isOpen;
        
        var spring = _hinge.spring;
        spring.spring = _springForce;
        spring.damper = _damping;
        spring.targetPosition = _isOpen ? _openAngle : 0f;
        _hinge.spring = spring;
        _hinge.useSpring = true;
    }
}

7. Layer Collision Matrix

Configure which layers collide in Edit → Project Settings → Physics:

code

Layer Setup:
  Default (0)     — General objects
  Player (9)      — Player character
  Enemy (10)      — Enemy characters
  Projectile (11) — Bullets, arrows
  Pickup (12)     — Collectibles
  Trigger (13)    — Trigger zones
  
Collision Matrix:
                Default  Player  Enemy  Projectile  Pickup  Trigger
  Default         ✓        ✓       ✓       ✓         —        —
  Player          ✓        —       ✓       —         ✓        ✓
  Enemy           ✓        ✓       —       ✓         —        ✓
  Projectile      ✓        —       ✓       —         —        —
  Pickup          —        ✓       —       —         —        —
  Trigger         —        ✓       ✓       —         —        —

Disabling unnecessary collisions is one of the cheapest physics optimizations.

8. 2D Physics

Unity uses Box2D for 2D physics. The API mirrors 3D but uses 2D suffix.

csharp

// 2D equivalents
Rigidbody2D rb2d;
BoxCollider2D boxCollider;
CircleCollider2D circleCollider;

// 2D raycasting
RaycastHit2D hit = Physics2D.Raycast(origin, direction, distance, layerMask);
if (hit.collider != null)
{
    Debug.Log($"Hit: {hit.collider.name}");
}

// 2D overlap
Collider2D[] results = Physics2D.OverlapCircleAll(center, radius, layerMask);

// 2D callbacks
private void OnCollisionEnter2D(Collision2D collision) { }
private void OnTriggerEnter2D(Collider2D other) { }

Important: 3D and 2D physics are completely separate systems. A 3D Raycast will never hit a 2D Collider and vice versa.

Best Practices

•All physics operations in FixedUpdate() — forces, movement, velocity changes.
•Use NonAlloc variants for queries in hot paths — RaycastNonAlloc, OverlapSphereNonAlloc.
•Use the Layer Collision Matrix — disable unnecessary collisions for free performance.
•Never move static colliders — always add a Rigidbody (set kinematic if needed).
•Use Rigidbody.interpolation = Interpolate for player-visible objects to prevent jitter.
•Set ContinuousDynamic collision detection on fast-moving objects (bullets, vehicles).
•Use QueryTriggerInteraction.Ignore on raycasts that shouldn't hit triggers.
•Prefer primitive colliders — compound several primitives instead of using MeshColliders.
•Cache raycast results in buffers — pre-allocate arrays and reuse them.
•Use Physics.Simulate() for prediction — simulate physics ahead for AI trajectory planning.

Common Pitfalls

Pitfall	Why It Hurts	Fix
Moving colliders without Rigidbody	Forces PhysX spatial tree rebuild every frame	Add Rigidbody (kinematic if needed)
Physics in `Update()`	Inconsistent results, frame-rate dependent	Use `FixedUpdate()`
Using `RaycastAll` in `Update()`	Allocates new array every frame	Use `RaycastNonAlloc`
Not using layers	Everything collides with everything	Set up Layer Collision Matrix
`Discrete` collision detection on fast objects	Bullets pass through walls (tunneling)	Use `ContinuousDynamic`
Mixing 2D and 3D physics	They don't interact at all	Choose one per game, or use separate layers
Scaling colliders via transform	Can cause physics instability	Scale the collider component, not the transform
Too many mesh colliders	Physics performance tanks	Use compound primitive colliders

05 Physics And Collision

Physics & Collision

Description

When To Use

Prerequisites

Instructions

1. Rigidbody Fundamentals

3D Rigidbody

Body Types

Applying Forces

2. Colliders

Collider Types

Compound Colliders

Trigger Colliders

3. Collision Callbacks

Collision Matrix

Collision Events

4. Raycasting

Basic Raycasts

SphereCast (Thicker raycast — more forgiving aim)

RaycastAll and RaycastNonAlloc

Overlap Queries (Area detection)

5. Physics Materials

6. Joints

Hinge Joint Example (Door)

7. Layer Collision Matrix

8. 2D Physics

Best Practices

Common Pitfalls

Reference