Unity 6.3 — Physics Module Reference¶

Last verified: 2026-02-13 Knowledge Gap: Unity 6 physics improvements, solver changes

Overview¶

Unity 6.3 uses PhysX 5.1 (improved from PhysX 4.x in 2022 LTS): - Better solver stability - Improved performance - Enhanced collision detection

Key Changes from 2022 LTS¶

Default Solver Iterations Increased¶

Unity 6 increased default solver iterations for better stability:

// Default changed from 6 to 8 iterations
Physics.defaultSolverIterations = 8; // Check if relying on old behavior

Enhanced Collision Detection¶

// ✅ Unity 6: Improved Continuous Collision Detection (CCD)
rigidbody.collisionDetectionMode = CollisionDetectionMode.ContinuousDynamic;
// Better handling of fast-moving objects

Core Physics Components¶

Rigidbody¶

// ✅ Best practice: Use AddForce, not direct velocity writes
Rigidbody rb = GetComponent<Rigidbody>();
rb.AddForce(Vector3.forward * 10f, ForceMode.Impulse);

// ❌ Avoid: Direct velocity assignment (can cause instability)
rb.velocity = new Vector3(0, 10, 0); // Only use when necessary

Colliders¶

// Primitive colliders: Box, Sphere, Capsule (cheapest)
// Mesh colliders: Expensive, use only for static geometry

// ✅ Compound colliders (multiple primitives) > single mesh collider

Raycasting¶

Efficient Raycasting (Avoid Allocations)¶

// ✅ Non-allocating raycast
if (Physics.Raycast(origin, direction, out RaycastHit hit, maxDistance)) {
    Debug.Log($"Hit: {hit.collider.name}");
}

// ✅ Multiple hits (non-allocating)
RaycastHit[] results = new RaycastHit[10];
int hitCount = Physics.RaycastNonAlloc(origin, direction, results, maxDistance);
for (int i = 0; i < hitCount; i++) {
    Debug.Log($"Hit {i}: {results[i].collider.name}");
}

// ❌ Avoid: RaycastAll (allocates array every call)
RaycastHit[] hits = Physics.RaycastAll(origin, direction); // GC allocation!

LayerMask for Selective Raycasting¶

// ✅ Use LayerMask to filter collisions
int layerMask = 1 << LayerMask.NameToLayer("Enemy");
Physics.Raycast(origin, direction, out RaycastHit hit, maxDistance, layerMask);

Physics Queries¶

OverlapSphere (Check for nearby objects)¶

// ✅ Non-allocating version
Collider[] results = new Collider[10];
int count = Physics.OverlapSphereNonAlloc(center, radius, results);
for (int i = 0; i < count; i++) {
    // Process results[i]
}

SphereCast (Thick raycast)¶

// Useful for character controllers
if (Physics.SphereCast(origin, radius, direction, out RaycastHit hit, maxDistance)) {
    // Hit something with a sphere-shaped ray
}

Collision Events¶

OnCollisionEnter / Stay / Exit¶

void OnCollisionEnter(Collision collision) {
    // Triggered when collision starts
    Debug.Log($"Collided with {collision.gameObject.name}");

    // Access contact points
    foreach (ContactPoint contact in collision.contacts) {
        Debug.DrawRay(contact.point, contact.normal, Color.red, 2f);
    }
}

OnTriggerEnter / Stay / Exit¶

void OnTriggerEnter(Collider other) {
    // Trigger collider (Is Trigger = true)
    if (other.CompareTag("Pickup")) {
        Destroy(other.gameObject);
    }
}

Character Controllers¶

CharacterController Component¶

CharacterController controller = GetComponent<CharacterController>();

// ✅ Move with collision detection
Vector3 move = transform.forward * speed * Time.deltaTime;
controller.Move(move);

// Apply gravity manually
if (!controller.isGrounded) {
    velocity.y += Physics.gravity.y * Time.deltaTime;
}
controller.Move(velocity * Time.deltaTime);

Physics Materials¶

Friction & Bounciness¶

// Create: Assets > Create > Physic Material
// Assign to collider: Collider > Material

// PhysicMaterial settings:
// - Dynamic Friction: 0.6 (sliding friction)
// - Static Friction: 0.6 (starting friction)
// - Bounciness: 0.0 - 1.0
// - Friction Combine: Average, Minimum, Maximum, Multiply
// - Bounce Combine: Average, Minimum, Maximum, Multiply

Joints¶

Fixed Joint (Attach two rigidbodies)¶

FixedJoint joint = gameObject.AddComponent<FixedJoint>();
joint.connectedBody = otherRigidbody;

Hinge Joint (Door, wheel)¶

HingeJoint hinge = gameObject.AddComponent<HingeJoint>();
hinge.axis = Vector3.up; // Rotation axis
hinge.useLimits = true;
hinge.limits = new JointLimits { min = -90, max = 90 };

Performance Optimization¶

Physics Layer Collision Matrix¶

Edit > Project Settings > Physics > Layer Collision Matrix - Disable unnecessary collision checks between layers - Massive performance gain

Fixed Timestep¶

Edit > Project Settings > Time > Fixed Timestep - Default: 0.02 (50 FPS physics) - Lower = more accurate, higher CPU cost - Match game's target framerate if possible

Simplified Collision Geometry¶

Use primitive colliders (box, sphere, capsule) over mesh colliders
Bake mesh colliders at build time, not runtime

Common Patterns¶

Ground Check (Character Controller)¶

bool IsGrounded() {
    float rayLength = 0.1f;
    return Physics.Raycast(transform.position, Vector3.down, rayLength);
}

Apply Explosion Force¶

void ApplyExplosion(Vector3 explosionPos, float radius, float force) {
    Collider[] colliders = Physics.OverlapSphere(explosionPos, radius);
    foreach (Collider hit in colliders) {
        Rigidbody rb = hit.GetComponent<Rigidbody>();
        if (rb != null) {
            rb.AddExplosionForce(force, explosionPos, radius);
        }
    }
}

Debugging¶

Physics Debugger (Unity 6+)¶

Window > Analysis > Physics Debugger
Visualize colliders, contacts, queries

Gizmos¶

void OnDrawGizmos() {
    Gizmos.color = Color.red;
    Gizmos.DrawWireSphere(transform.position, detectionRadius);
}

Sources¶

https://docs.unity3d.com/6000.0/Documentation/Manual/PhysicsOverview.html
https://docs.unity3d.com/ScriptReference/Physics.html