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 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.