Tags: unity, rigidbody, collision, trigger, physics, layer-matrix, impulses, game-dev Last updated: 2026-06-27

Rigidbody & Collision Cheatsheet

Quick Reference

Concept	Key Point
Collider	Defines shape for collision detection
Rigidbody	Makes object respond to physics forces
Trigger	Collider with `Is Trigger = true` — detects overlap, no physical response
Kinematic	Rigidbody with `Is Kinematic = true` — script-driven, ignores forces
Continuous Detection	Prevents tunnelling for fast-moving objects

Rigidbody Component

Key Properties

Property	Description
Mass	Affects collision response (heavier = harder to push)
Drag	Air resistance (0 = none, higher = decelerates faster)
Angular Drag	Rotational resistance
Use Gravity	Enable/disable gravity on this object
Is Kinematic	Script controls transform; ignores forces and collisions with static colliders
Interpolate	Smooths transform between physics steps (None / Interpolate / Extrapolate)
Collision Detection	Discrete / Continuous / Continuous Dynamic / Continuous Speculative
Constraints	Freeze position/rotation on X, Y, Z axes

Force Application Modes

Mode	Use Case
`Force`	Continuous force (mass-dependent, applied over time)
`Acceleration`	Continuous acceleration (ignores mass)
`Impulse`	Instant force (mass-dependent, e.g. jump, explosion)
`VelocityChange`	Instant velocity change (ignores mass)

# Apply force examples
rb.AddForce(Vector3.forward * 10f, ForceMode.Force);
rb.AddForce(Vector3.up * 500f, ForceMode.Impulse);     # Jump
rb.AddForce(explosionDir * 1000f, ForceMode.Impulse);   # Explosion

Collider Types

Type	Shape	Best For	Performance
BoxCollider	Rectangular	Walls, floors, crates	Fast
SphereCollider	Sphere	Rolling objects, triggers	Fast
CapsuleCollider	Capsule	Character bodies	Medium
MeshCollider	Arbitrary mesh	Complex static geometry	Slow (Convex = faster)
WheelCollider	Specialised	Vehicle wheels	Medium
TerrainCollider	Heightmap	Terrain surfaces	Medium

Collider Tips

Primitive colliders (box, sphere, capsule) are always faster than mesh colliders.
MeshCollider.Convex = true enables collision with other convex mesh colliders (max 255 triangles).
Compound colliders — use multiple primitive colliders on one GameObject for complex shapes (cheaper than MeshCollider).
Collider size — avoid micro-colliders (<0.01 units); PhysX has a minimum contact offset.

Triggers vs Colliders

Aspect	Collider	Trigger
Physical response	Yes (bounce, push, stop)	No (overlap only)
Callback	`OnCollisionEnter/Stay/Exit`	`OnTriggerEnter/Stay/Exit`
Use case	Walls, floors, physics objects	Pickup zones, damage areas, checkpoints
Setting	`Is Trigger = false`	`Is Trigger = true`

Trigger Callbacks

void OnTriggerEnter(Collider other)  { }  # First frame of overlap
void OnTriggerStay(Collider other)   { }  # Every frame during overlap
void OnTriggerExit(Collider other)   { }  # First frame after separation

Collision Callbacks

void OnCollisionEnter(Collision collision) { }
  # collision.contacts[] — array of contact points
  # collision.relativeVelocity — impact speed
  # collision.impulse — total impulse applied

void OnCollisionStay(Collision collision)  { }
void OnCollisionExit(Collision collision)  { }

Collision Detection Modes

Mode	Description	When to Use
Discrete	Checks at fixed intervals	Slow-moving objects
Continuous	Sweeps collider against static geometry	Fast-moving player vs static world
Continuous Dynamic	Sweeps against all colliders	Fast-moving dynamic objects (bullets)
Continuous Speculative	Broad-phase sweep, then speculative contacts	Kinematic bodies, many fast objects

Tunnelling Prevention

Tunnelling — object moves so fast it passes through a collider between physics steps.
Fix: use Continuous / Continuous Dynamic detection.
Alt fix: increase physics timestep (Edit → Project Settings → Time → Fixed Timestep, default 0.02s).
Alt fix: raycast ahead of movement (manual sweep test).

Layer-Based Collision Matrix

Configure in Edit → Project Settings → Physics → Layer Collision Matrix.

Pattern	Setup
Player ignores other players	Untick Player/Player cell
Bullets only hit enemies + walls	Tick Bullet/Enemy + Bullet/Environment
Triggers ignore visuals	Tick Trigger/Player only

# Check layer at runtime
if (other.gameObject.layer == LayerMask.NameToLayer("Enemy")) { }

# Raycast with layer mask
int mask = LayerMask.GetMask("Ground", "Wall");
Physics.Raycast(origin, direction, out hit, 100f, mask);

Physics Materials

Property	Description
Dynamic Friction	Friction while moving (0–1)
Static Friction	Friction to start moving (0–1)
Bounciness	Restitution (0 = no bounce, 1 = perfect bounce)
Friction Combine	Average / Minimum / Maximum / Multiply
Bounce Combine	Average / Minimum / Maximum / Multiply

# Create via code
PhysicMaterial mat = new PhysicMaterial();
mat.dynamicFriction = 0.3f;
mat.staticFriction = 0.5f;
mat.bounciness = 0.8f;
GetComponent<Collider>().material = mat;

Common Patterns

Explosion Force

Vector3 origin = transform.position;
float radius = 5f;
float power = 1000f;
Collider[] hits = Physics.OverlapSphere(origin, radius);
foreach (Collider hit in hits) {
    Rigidbody rb = hit.GetComponent<Rigidbody>();
    if (rb) rb.AddExplosionForce(power, origin, radius, 3f);
}

Raycast for Ground Check

bool isGrounded = Physics.Raycast(
    transform.position, Vector3.down, 0.1f, groundMask);

MovePosition (Kinematic)

Rigidbody rb = GetComponent<Rigidbody>();
rb.MovePosition(rb.position + velocity * Time.fixedDeltaTime);
rb.MoveRotation(Quaternion.Euler(0, rotation, 0));
# Always use MovePosition/MoveRotation in FixedUpdate

Performance Tips

Use primitive colliders over MeshCollider where possible.
Disable Rigidbody when not needed (e.g. Sleep()).
Reduce solverIterations on low-priority Rigidbodies.
Use layers to minimise unnecessary collision pairs.
Batch static colliders with Static flag (static batching).