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 6.3 LTS rename: Rigidbody.velocity is now Rigidbody.linearVelocity (the old name is deprecated). Code copied from older tutorials will warn or fail to compile.

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. 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. A collision needs a Collider on both, and at least one Rigidbody.
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, this one has a (non-kinematic) Rigidbody.
private void OnCollisionEnter(Collision col)
{
    Debug.Log($"Hit {col.gameObject.name} at {col.contacts[0].point}");
}

// Overlap: one collider has 'Is Trigger' = true. Requires a Rigidbody on at least one party.
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

• Rigidbody.velocity doesn't exist in Unity 6.3 LTS — use linearVelocity (and angularVelocity is unchanged).
• 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.
• Physics.Raycast ignores triggers by default — a ray won't report a trigger collider unless you pass QueryTriggerInteraction.Collide (or flip the global Physics.queriesHitTriggers / Project Settings → Physics → Queries Hit Triggers). It also returns false when the ray origin starts inside the target collider.
• 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.
• 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.
• 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 "Introduction to rigid body physics" (sleeping) 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.