unity-physics

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

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

2. Collision vs trigger callbacks

3. Ground check with a layer-masked raycast

4. Kinematic platform that still pushes bodies

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.
• 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.