Unity Physics Agent Skill

Unity Physics (Rigidbody / PhysX)

Make objects move, collide, and detect each other with Unity 6's built-in 3D physics

(PhysX). Get the FixedUpdate discipline, trigger-vs-collision rules, and collision layers

right. Targets Unity 6 (6000.0 LTS).

> Unity 6 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

• 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.

• Do all physics in FixedUpdate. Read input in Update, store intent, then apply

forces / set linearVelocity / call MovePosition in FixedUpdate.

• 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).

• Pick collision vs trigger. A solid collision blocks and calls OnCollisionEnter; a

Collider with Is Trigger checked passes through and calls OnTriggerEnter.

• 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.

• 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: 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 — use linearVelocity (and

angularVelocity is unchanged).

• Setting transform.position on a dynamic Rigidbody — teleports it, skips collision.

Use MovePosition (kinematic/interpolated) or apply forces.

• 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) — set the Rigidbody's Collision Detection

to Continuous (or Continuous Dynamic) for bullets/fast movers.

• Non-uniform-scaled MeshColliders or scaled colliders misbehave; prefer primitive

colliders and keep scale uniform.

• 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.

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.