Unity Physics (Rigidbody / PhysX)

Make objects move, collide, and detect each other with Unity 6.3 LTS's built-in 3D physics (PhysX). Get the FixedUpdate discipline, trigger-vs-collision rules, and collision layers right. Targets Unity 6.3 LTS (6000.3).

Unity 6 renames: Rigidbody.velocity → linearVelocity, drag → linearDamping, angularDrag → angularDamping. The old names still compile in 6.3 LTS but are obsolete (warnings; the API Updater offers the rename), so write the new names. PhysicMaterial → PhysicsMaterial is a hard compile error in old code.

When to use

• Use when giving an object physical motion (forces, velocity, gravity), responding to collisions or triggers, setting up collision layers/masks, raycasting for ground checks or line-of-sight, or connecting bodies with joints.
• Use when scenes/prefabs contain Rigidbody + Collider components.

When not to use: 2D physics (Rigidbody2D, Collider2D) is a separate API — adapt the concepts but the types differ. Unity 6.3 also has a separate low-level 2D API (UnityEngine.LowLevelPhysics2D: PhysicsWorld, PhysicsBody) with its own isolated worlds; it is not the Rigidbody2D system. Cross-engine feel tuning (timestep, jitter, tunnelling) → physics-tuning. Reading input that drives movement → unity-input-system.

Core workflow

1. Add a Rigidbody to anything that should be simulated; add a Collider to anything that should be hit. OnCollision* messages need a Collider on both and at least one dynamic (non-kinematic) Rigidbody: with the default Contact Pairs Mode, kinematic vs kinematic and kinematic vs static send no collision messages.
2. Do all physics in FixedUpdate. Read input in Update, store intent, then apply forces / set linearVelocity / call MovePosition in FixedUpdate.
3. Move bodies through the physics API, not the Transform. Use AddForce, linearVelocity, or MovePosition — never assign transform.position to a non-kinematic Rigidbody (it teleports and breaks collision resolution).
4. Pick collision vs trigger. A solid collision blocks and calls OnCollisionEnter; a Collider with Is Trigger checked passes through and calls OnTriggerEnter.
5. Organise interactions with layers. Put objects on layers and edit the Layer Collision Matrix (Project Settings → Physics) so unrelated things don't test against each other.
6. Verify with the Physics Debugger (Window → Analysis → Physics Debugger) and by watching for jitter; if fast objects pass through walls, raise Collision Detection mode.

Patterns

1. Force-based movement in FixedUpdate (with a speed clamp)

using UnityEngine;

[RequireComponent(typeof(Rigidbody))]
public class Mover : MonoBehaviour
{
    [SerializeField] private float accel = 30f, maxSpeed = 8f;
    private Rigidbody _rb;
    private Vector3 _input;   // set from Update / input system

    private void Awake() => _rb = GetComponent<Rigidbody>();

    private void FixedUpdate()
    {
        _rb.AddForce(_input * accel, ForceMode.Acceleration);     // mass-independent accel
        // Unity 6.3 LTS: linearVelocity (was 'velocity'). Clamp horizontal speed.
        Vector3 flat = new(_rb.linearVelocity.x, 0, _rb.linearVelocity.z);
        if (flat.magnitude > maxSpeed)
        {
            flat = flat.normalized * maxSpeed;
            _rb.linearVelocity = new Vector3(flat.x, _rb.linearVelocity.y, flat.z);
        }
    }
}

ForceMode: Force (continuous, mass-scaled), Acceleration (continuous, ignores mass), Impulse (instant, mass-scaled — jumps), VelocityChange (instant, ignores mass).

2. Collision vs trigger callbacks

// Solid hit: both have colliders, at least one has a dynamic (non-kinematic) Rigidbody.
private void OnCollisionEnter(Collision col)
{
    // GetContact avoids the garbage that the col.contacts array allocates.
    Debug.Log($"Hit {col.gameObject.name} at {col.GetContact(0).point}");
}

// Overlap: one collider has 'Is Trigger' = true, and at least one party has a Rigidbody
// (or a Rigidbody carries the trigger). Two static colliders never report.
private void OnTriggerEnter(Collider other)
{
    if (other.CompareTag("Pickup")) Destroy(other.gameObject);
}

3. Ground check with a layer-masked raycast

[SerializeField] private LayerMask groundMask;   // set to your "Ground" layer in the Inspector

private bool IsGrounded()
{
    // Cast a short ray down; only test colliders on groundMask.
    return Physics.Raycast(transform.position, Vector3.down, out RaycastHit hit,
                           1.1f, groundMask);
}

4. Kinematic platform that still pushes bodies

// isKinematic Rigidbody: not driven by forces, but MovePosition interpolates and carries
// resting bodies correctly (unlike moving the Transform directly).
private void FixedUpdate() => _rb.MovePosition(_rb.position + Vector3.right * (2f * Time.fixedDeltaTime));

Pitfalls

• Obsolete warnings on velocity / drag / angularDrag — Unity 6 renamed them to linearVelocity, linearDamping and angularDamping (angularVelocity is unchanged). Let the API Updater rename them or edit by hand; PhysicMaterial must become PhysicsMaterial.
• Setting transform.position on a dynamic Rigidbody — teleports it, skips collision. Use MovePosition (kinematic/interpolated) or apply forces. If you do write the transform, physics queries (Raycast, OverlapSphere) see the old position until the next physics step — call Physics.SyncTransforms() once before a same-frame query, never every frame.
• OnCollisionEnter never fires on a CharacterController — CharacterController.Move bypasses the Rigidbody system; it reports hits via OnControllerColliderHit(ControllerColliderHit) instead. Don't add a Rigidbody to "fix" it — the two are mutually exclusive movement modes.
• A ground or line-of-sight ray stops on a trigger volume — Physics.Raycast hits trigger colliders by default (Project Settings → Physics → Queries Hit Triggers is on, i.e. Physics.queriesHitTriggers == true). Pass QueryTriggerInteraction.Ignore to skip them in one query, or turn the global setting off. A ray also returns false for a collider its origin starts inside.
• A resting Rigidbody stays put after you move or disable what it rests on — below the Sleep Threshold a body goes to sleep. Collisions and AddForce wake it automatically, but moving a static collider (no Rigidbody) via its Transform may not, so the crate hangs in mid-air when the floor slides away. Call Rigidbody.WakeUp() on the affected bodies.
• Applying forces in Update — frame-rate-dependent and jittery. Physics goes in FixedUpdate.
• OnCollisionEnter never fires between two kinematic bodies (or kinematic vs static) — with the default Contact Pairs Mode, Unity only sends collision messages when at least one body is dynamic. Make one dynamic, use a trigger, or set Project Settings → Physics → Contact Pairs Mode to enable kinematic pairs (more contact work every step).
• Trigger callbacks never fire — triggers need a Rigidbody on at least one of the two colliders, and both colliders enabled; two static triggers don't report overlaps.
• Fast objects pass through walls (tunnelling) — raise the Rigidbody's Collision Detection from Discrete. Note Continuous sweeps against static colliders only (it falls back to Discrete against other dynamic bodies); Continuous Dynamic also sweeps against other continuous dynamic bodies; Continuous Speculative works against everything and is cheaper. For bullets, also consider a SphereCast/Raycast along the trajectory instead of a collider.
• Non-uniform-scaled MeshColliders or scaled colliders misbehave; prefer primitive colliders and keep scale uniform.
• A concave MeshCollider on a moving body — Mesh colliders are concave by default, and concave ones can only be static or kinematic; two concave colliders never collide at all. For a dynamic Rigidbody enable Convex on the MeshCollider, or build a compound collider from primitives.
• Part of a compound body drifts off on its own — a compound collider should have exactly one Rigidbody, on the root. An extra Rigidbody on a child makes that child a separate body.
• Nothing moves and no callbacks fire, but raycasts still hit — Time.timeScale = 0 stops FixedUpdate and the simulation; queries read collider geometry and keep working. Restore a positive timeScale when the pause ends.
• Everything collides with everything — wasted cost; assign layers and prune the Layer Collision Matrix.

References

• For raycast variants (SphereCast, RaycastAll, OverlapSphere, LayerMask bit math) and joints (FixedJoint, HingeJoint, ConfigurableJoint, breakable joints), read references/raycasting-and-joints.md.
• Primary docs: Unity Manual "Physics" section and ScriptReference/Rigidbody, ScriptReference/Physics.Raycast. For the added gotchas: ScriptReference/CollisionDetectionMode, ScriptReference/Physics-queriesHitTriggers, ScriptReference/MonoBehaviour.OnControllerColliderHit, ScriptReference/Physics.SyncTransforms, ScriptReference/Rigidbody.WakeUp, and the Manual pages "Physics settings" (Queries Hit Triggers, Contact Pairs Mode), "Introduction to rigid body physics" (sleeping), "Introduction to compound colliders" and "Introduction to Mesh colliders" (concave vs convex).

Related skills

• physics-tuning — engine-agnostic feel: fixed timestep, mass/drag, CCD, stability.
• unity-csharp-scripting — the FixedUpdate/Update split these patterns rely on.
• unity-navmesh — agent movement that is not force-driven.