# Roblox Physics

> Roblox rigid-body physics for vehicles, mechanisms, doors, platforms, and dynamic objects. Covers assemblies, root parts, anchoring, WeldConstraint vs RigidConstraint, mechanical constraints (hinge, spring, prismatic, rope), mover constraints (AlignPosition, LinearVelocity, VectorForce), network ownership, collision filtering, the sleep system, adaptive timestepping, and units. Use for anything that moves or connects under physics simulation.

- Skill: `nonlooped/roblox-physics` (Agent Skill, multi-file: 10 files)
- Install (CLI): `npx skillmds@latest add nonlooped/roblox-physics`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nonlooped/roblox-physics/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: nonlooped (https://skillmd.com/u/nonlooped)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nonlooped/roblox-physics

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# roblox-physics

**Official sources (always check these for the latest):**
- https://create.roblox.com/docs/physics
- https://create.roblox.com/docs/physics/assemblies
- https://create.roblox.com/docs/physics/mechanical-constraints
- https://create.roblox.com/docs/physics/mover-constraints
- https://create.roblox.com/docs/physics/network-ownership
- https://create.roblox.com/docs/physics/sleep-system
- https://create.roblox.com/docs/physics/adaptive-timestepping
- https://create.roblox.com/docs/physics/units
- https://create.roblox.com/docs/workspace/collisions

This skill covers the modern constraint-based physics system, not deprecated `BodyMover` objects. It focuses on building correct, stable, multiplayer-safe mechanisms.

## When to use this skill

Activate when:
- Building vehicles, doors, elevators, cranes, swings, suspension, or platforms.
- Moving objects with forces instead of setting `CFrame` every frame.
- Tuning stability for complex mechanisms or multiplayer physics.
- Choosing between mechanical and mover constraints.
- Debugging why assemblies sleep, jitter, or behave unexpectedly.

Cross-reference:
- [roblox-networking/SKILL.md](../roblox-networking/SKILL.md) for network ownership security and server-authoritative validation.
- [roblox-core/SKILL.md](../roblox-core/SKILL.md) for services and script locations.
- [roblox-testing/SKILL.md](../roblox-testing/SKILL.md) for profiling physics with MicroProfiler.

## Core concepts

### Assemblies

An **assembly** is one or more parts connected by rigid welds or movable joints, simulated as a single rigid body.

Key `BasePart` properties (same for any part in the assembly):
- `AssemblyLinearVelocity` / `AssemblyAngularVelocity` — prefer constraints or `ApplyImpulse` over direct assignment for realistic motion.
- `AssemblyCenterOfMass` — force here produces pure linear acceleration.
- `AssemblyMass` — sum of all part masses; infinite if any part is anchored.
- `AssemblyRootPart` — automatically chosen root for replication and network ownership.

Root-part priority: anchored > non-massless > higher `RootPriority` > size/name heuristics.

### Anchoring

- Anchoring one part in an assembly makes it the root; the rest are implicitly anchored.
- Anchoring multiple parts in the same assembly **splits** it.
- To anchor an entire assembly, only anchor the root part.

### Collision filtering

Use collision groups (`PhysicsService:RegisterCollisionGroup`, `BasePart.CollisionGroup`) or `NoCollisionConstraint` for part-to-part filtering. `CanCollide`, `CanTouch`, and `CanQuery` control different behaviors; see [references/collisions-and-filtering.md](references/collisions-and-filtering.md).

## Mechanical constraints

All mechanical constraints connect one or two `Attachment`s (or `Bone`s), except `WeldConstraint` and `NoCollisionConstraint` which use `Part0`/`Part1`.

| Constraint | Use case |
| --- | --- |
| `WeldConstraint` | Rigidly lock two parts together, same relative transform |
| `RigidConstraint` | Same as WeldConstraint but attachment-based, supports bones |
| `HingeConstraint` | Doors, levers, rotating parts; motor/servo optional |
| `PrismaticConstraint` | Sliding doors, elevators, pistons |
| `CylindricalConstraint` | Slides + rotates, like hydraulic rams or landing gear |
| `SpringConstraint` | Springs, shocks, suspension |
| `TorsionSpringConstraint` | Rotational springs |
| `BallSocketConstraint` | Shoulders, ball joints |
| `UniversalConstraint` | Drive shafts, constant-velocity joints |
| `RopeConstraint` | Cables, winches, maximum length |
| `RodConstraint` | Fixed separation distance |
| `PlaneConstraint` | Constrain motion to a plane |
| `NoCollisionConstraint` | Disable collisions between two specific parts |

See [references/mechanical-constraints.md](references/mechanical-constraints.md) for creation, orientation, motor/servo tuning, and limits.

## Mover constraints

Modern replacements for deprecated `BodyMover`s:

| Legacy | Modern | Use case |
| --- | --- | --- |
| `BodyPosition` | `AlignPosition` | Move attachment to a position or another attachment |
| `BodyGyro` | `AlignOrientation` | Align orientation; `LookAtPosition` for tracking |
| `BodyVelocity` | `LinearVelocity` | Maintain constant velocity along vector/line/plane |
| `BodyAngularVelocity` | `AngularVelocity` | Maintain constant angular velocity |
| `BodyForce`/`BodyThrust` | `VectorForce` | Apply constant force |
| `RocketPropulsion` | `LineForce` + `AlignOrientation` | Follow + face target |
| — | `Torque` | Apply constant torque |
| — | `LineForce` | Force along line between two attachments |
| — | `AnimationConstraint` | Constraint driven by animation/transform |

See [references/mover-constraints.md](references/mover-constraints.md) for force modes (`Magnitude` vs `PerAxis`), relativity frames, rigidity, and reaction forces.

## Network ownership

- The server owns anchored parts.
- Unanchored parts near a player character are automatically owned by that client.
- Set ownership explicitly with `BasePart:SetNetworkOwner(player)` (server only). Reset with `SetNetworkOwnershipAuto()`.
- Assign vehicle/driver ownership carefully: the first seated player may own the whole assembly otherwise.

**Security:** clients can exploit owned parts (teleport, fake collisions). Validate gameplay-critical events server-side. See [references/network-ownership.md](references/network-ownership.md).

## Sleep system

Assemblies stop simulating when still to save performance. They wake on collisions, property changes, impulses, or gravity/wind changes.

- If a slow mechanism falls asleep, increase motion or use actuated joints (motor/servo constraints) which get stricter sleep thresholds.
- Visualize sleep states with **Awake parts** in Studio's Visualization Options.

## Physics stepping method

Workspace.PhysicsSteppingMethod:
- **Fixed** (default, 240 Hz) — best general choice; use for racing, destruction, tanks, or when most parts already solve at 240 Hz.
- **Adaptive** — assigns assemblies to 60/120/240 Hz islands for up to ~2.5× performance in suitable experiences.

Use the MicroProfiler to check island distribution.

## Units quick reference

| Roblox | Metric |
| --- | --- |
| 1 stud | 28 cm |
| 1 RMU | 21.952 kg |
| 196.2 studs/s² | default "Classic" gravity |
| 35 studs/s² | "Realistic" gravity (≈ 9.8 m/s²) |

See [references/units-and-physical-properties.md](references/units-and-physical-properties.md).

## Common mistakes this skill prevents

- Using deprecated `BodyMover`s instead of modern constraints.
- Setting `CFrame` every frame instead of using forces/constraints.
- Anchoring every part in an assembly (splits it and hurts performance).
- Ignoring network ownership on vehicles and then wondering why input lags.
- Trusting `Touched` events from client-owned parts for damage/authority.
- Letting actuated joints fall asleep prematurely.

## Scripts

- `scripts/VehicleController.lua` — chassis setup with `HingeConstraint` steering and motor drive, with client ownership and server validation.
- `scripts/DoorHinge.lua` — motorized/servo door with limits and state machine.
- `scripts/PlatformMover.lua` — `AlignPosition` + `AlignOrientation` platform with configurable waypoints.
- `scripts/Suspension.lua` — spring-damper suspension using `SpringConstraint`.

## How to proceed

1. Decide if the object is a rigid mechanism (mechanical constraint) or force-driven (mover constraint).
2. Plan assemblies, root parts, and anchoring strategy.
3. Set network ownership for multiplayer responsiveness.
4. Tune forces/velocities using units and physical properties.
5. Profile with MicroProfiler and watch sleep/ownership visualizations.

<!-- catalog:references:start -->
## Reference index

- [collisions-and-filtering.md](references/collisions-and-filtering.md)
- [mechanical-constraints.md](references/mechanical-constraints.md)
- [mover-constraints.md](references/mover-constraints.md)
- [network-ownership.md](references/network-ownership.md)
- [units-and-physical-properties.md](references/units-and-physical-properties.md)
<!-- catalog:references:end -->