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.