--- name: roblox-physics description: "Build or debug Roblox vehicles, mechanisms, and moving objects. Use for assemblies, mechanical or mover constraints, network ownership, collision filtering, and simulation performance." last_reviewed: 2026-06-17 --- # roblox-physics **Official sources:** - 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 Use mechanical constraints to connect parts and mover constraints to apply forces or velocity targets. For older code, use the BodyMover migration table below. ## 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 (`Workspace:RegisterCollisionGroup` (inherited from `WorldRoot`), `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 - 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. ## Reference index - [collisions-and-filtering.md](references/collisions-and-filtering.md): Configure collision groups, touch events, or spatial query filters. - [mechanical-constraints.md](references/mechanical-constraints.md): Connect parts with hinges, springs, ropes, or other mechanical constraints. - [mover-constraints.md](references/mover-constraints.md): Apply forces or velocity targets, or migrate BodyMover code. - [network-ownership.md](references/network-ownership.md): Assign physics simulation to the server or a player and debug ownership. - [units-and-physical-properties.md](references/units-and-physical-properties.md): Tune mass, density, friction, force, or simulation stepping.